diff --git a/lib/bundler/dsl.rb b/lib/bundler/dsl.rb
index 4f9fbc55b1f2e7..0e36f52269abfa 100644
--- a/lib/bundler/dsl.rb
+++ b/lib/bundler/dsl.rb
@@ -290,7 +290,7 @@ def add_dependency(name, version = nil, options = {})
             @dependencies.delete(current)
           elsif dep.gemspec_dev_dep?
             return
-          elsif current.source != dep.source
+          elsif current.source.to_s != dep.source.to_s
             raise GemfileError, "You cannot specify the same gem twice coming from different sources.\n" \
                             "You specified that #{name} (#{dep.requirement}) should come from " \
                             "#{current.source || "an unspecified source"} and #{dep.source}\n"
diff --git a/lib/bundler/source/gemspec.rb b/lib/bundler/source/gemspec.rb
index b59dce1d09055b..ed766dbe74b5f4 100644
--- a/lib/bundler/source/gemspec.rb
+++ b/lib/bundler/source/gemspec.rb
@@ -10,6 +10,10 @@ def initialize(options)
         super
         @gemspec = options["gemspec"]
       end
+
+      def to_s
+        "gemspec at `#{@path}`"
+      end
     end
   end
 end
diff --git a/lib/bundler/source/path.rb b/lib/bundler/source/path.rb
index 7511def2306192..76bd1c66c19f08 100644
--- a/lib/bundler/source/path.rb
+++ b/lib/bundler/source/path.rb
@@ -53,6 +53,8 @@ def to_s
         "source at `#{@path}`"
       end
 
+      alias_method :identifier, :to_s
+
       alias_method :to_gemfile, :path
 
       def hash
diff --git a/lib/rubygems/commands/sources_command.rb b/lib/rubygems/commands/sources_command.rb
index 9e2f156da374ce..7e5c2a2465e6f3 100644
--- a/lib/rubygems/commands/sources_command.rb
+++ b/lib/rubygems/commands/sources_command.rb
@@ -18,6 +18,14 @@ def initialize
       options[:add] = value
     end
 
+    add_option "--append SOURCE_URI", "Append source (can be used multiple times)" do |value, options|
+      options[:append] = value
+    end
+
+    add_option "-p", "--prepend SOURCE_URI", "Prepend source (can be used multiple times)" do |value, options|
+      options[:prepend] = value
+    end
+
     add_option "-l", "--list", "List sources" do |value, options|
       options[:list] = value
     end
@@ -26,8 +34,7 @@ def initialize
       options[:remove] = value
     end
 
-    add_option "-c", "--clear-all",
-               "Remove all sources (clear the cache)" do |value, options|
+    add_option "-c", "--clear-all", "Remove all sources (clear the cache)" do |value, options|
       options[:clear_all] = value
     end
 
@@ -68,6 +75,60 @@ def add_source(source_uri) # :nodoc:
     end
   end
 
+  def append_source(source_uri) # :nodoc:
+    check_rubygems_https source_uri
+
+    source = Gem::Source.new source_uri
+
+    check_typo_squatting(source)
+
+    begin
+      source.load_specs :released
+      was_present = Gem.sources.include?(source)
+      Gem.sources.append source
+      Gem.configuration.write
+
+      if was_present
+        say "#{source_uri} moved to end of sources"
+      else
+        say "#{source_uri} added to sources"
+      end
+    rescue Gem::URI::Error, ArgumentError
+      say "#{source_uri} is not a URI"
+      terminate_interaction 1
+    rescue Gem::RemoteFetcher::FetchError => e
+      say "Error fetching #{Gem::Uri.redact(source.uri)}:\n\t#{e.message}"
+      terminate_interaction 1
+    end
+  end
+
+  def prepend_source(source_uri) # :nodoc:
+    check_rubygems_https source_uri
+
+    source = Gem::Source.new source_uri
+
+    check_typo_squatting(source)
+
+    begin
+      source.load_specs :released
+      was_present = Gem.sources.include?(source)
+      Gem.sources.prepend source
+      Gem.configuration.write
+
+      if was_present
+        say "#{source_uri} moved to top of sources"
+      else
+        say "#{source_uri} added to sources"
+      end
+    rescue Gem::URI::Error, ArgumentError
+      say "#{source_uri} is not a URI"
+      terminate_interaction 1
+    rescue Gem::RemoteFetcher::FetchError => e
+      say "Error fetching #{Gem::Uri.redact(source.uri)}:\n\t#{e.message}"
+      terminate_interaction 1
+    end
+  end
+
   def check_typo_squatting(source)
     if source.typo_squatting?("rubygems.org")
       question = <<-QUESTION.chomp
@@ -147,14 +208,20 @@ def description # :nodoc:
 of them in your list.  https://rubygems.org is recommended as it brings the
 protections of an SSL connection to gem downloads.
 
-To add a source use the --add argument:
+To add a private gem source use the --prepend argument to insert it before
+the default source. This is usually the best place for private gem sources:
 
-    $ gem sources --add https://my.private.source
+    $ gem sources --prepend https://my.private.source
     https://my.private.source added to sources
 
 RubyGems will check to see if gems can be installed from the source given
 before it is added.
 
+To add or move a source after all other sources, use --append:
+
+    $ gem sources --append https://rubygems.org
+    https://rubygems.org moved to end of sources
+
 To remove a source use the --remove argument:
 
     $ gem sources --remove https://my.private.source/
@@ -182,6 +249,8 @@ def list # :nodoc:
 
   def list? # :nodoc:
     !(options[:add] ||
+      options[:prepend] ||
+      options[:append] ||
       options[:clear_all] ||
       options[:remove] ||
       options[:update])
@@ -190,11 +259,13 @@ def list? # :nodoc:
   def execute
     clear_all if options[:clear_all]
 
-    source_uri = options[:add]
-    add_source source_uri if source_uri
+    add_source options[:add] if options[:add]
+
+    prepend_source options[:prepend] if options[:prepend]
+
+    append_source options[:append] if options[:append]
 
-    source_uri = options[:remove]
-    remove_source source_uri if source_uri
+    remove_source options[:remove] if options[:remove]
 
     update if options[:update]
 
diff --git a/lib/rubygems/exceptions.rb b/lib/rubygems/exceptions.rb
index adf7ad6d7d05ac..362b09dcbb9949 100644
--- a/lib/rubygems/exceptions.rb
+++ b/lib/rubygems/exceptions.rb
@@ -23,15 +23,8 @@ def initialize(unknown_command)
   def self.attach_correctable
     return if defined?(@attached)
 
-    if defined?(DidYouMean::SPELL_CHECKERS) && defined?(DidYouMean::Correctable)
-      if DidYouMean.respond_to?(:correct_error)
-        DidYouMean.correct_error(Gem::UnknownCommandError, Gem::UnknownCommandSpellChecker)
-      else
-        DidYouMean::SPELL_CHECKERS["Gem::UnknownCommandError"] =
-          Gem::UnknownCommandSpellChecker
-
-        prepend DidYouMean::Correctable
-      end
+    if defined?(DidYouMean) && DidYouMean.respond_to?(:correct_error)
+      DidYouMean.correct_error(Gem::UnknownCommandError, Gem::UnknownCommandSpellChecker)
     end
 
     @attached = true
diff --git a/lib/rubygems/source_list.rb b/lib/rubygems/source_list.rb
index 33db64fbc1513d..19bf4595c4dc7d 100644
--- a/lib/rubygems/source_list.rb
+++ b/lib/rubygems/source_list.rb
@@ -59,6 +59,42 @@ def <<(obj)
     src
   end
 
+  ##
+  # Prepends +obj+ to the beginning of the source list which may be a Gem::Source, Gem::URI or URI
+  # Moves +obj+ to the beginning of the list if already present.
+  # String.
+
+  def prepend(obj)
+    src = case obj
+          when Gem::Source
+            obj
+          else
+            Gem::Source.new(obj)
+    end
+
+    @sources.delete(src) if @sources.include?(src)
+    @sources.unshift(src)
+    src
+  end
+
+  ##
+  # Appends +obj+ to the end of the source list, moving it if already present.
+  # +obj+ may be a Gem::Source, Gem::URI or URI String.
+  # Moves +obj+ to the end of the list if already present.
+
+  def append(obj)
+    src = case obj
+          when Gem::Source
+            obj
+          else
+            Gem::Source.new(obj)
+    end
+
+    @sources.delete(src) if @sources.include?(src)
+    @sources << src
+    src
+  end
+
   ##
   # Replaces this SourceList with the sources in +other+  See #<< for
   # acceptable items in +other+.
diff --git a/spec/bundler/commands/install_spec.rb b/spec/bundler/commands/install_spec.rb
index e7c1f5e456e2d2..77c7dff0ce38a8 100644
--- a/spec/bundler/commands/install_spec.rb
+++ b/spec/bundler/commands/install_spec.rb
@@ -615,6 +615,30 @@
       expect(err).to include("Two gemspec development dependencies have conflicting requirements on the same gem: rubocop (~> 1.36.0) and rubocop (~> 2.0). Bundler cannot continue.")
     end
 
+    it "errors out if a gem is specified in a gemspec and in the Gemfile" do
+      gem = tmp("my-gem-1")
+
+      build_lib "rubocop", path: gem do |s|
+        s.add_development_dependency "rubocop", "~> 1.0"
+      end
+
+      build_repo4 do
+        build_gem "rubocop"
+      end
+
+      gemfile <<~G
+        source "https://gem.repo4"
+
+        gem "rubocop", :path => "#{gem}"
+        gemspec path: "#{gem}"
+      G
+
+      bundle :install, raise_on_error: false
+
+      expect(err).to include("There was an error parsing `Gemfile`: You cannot specify the same gem twice coming from different sources.")
+      expect(err).to include("You specified that rubocop (>= 0) should come from source at `#{gem}` and gemspec at `#{gem}`")
+    end
+
     it "does not warn if a gem is added once in Gemfile and also inside a gemspec as a development dependency, with same requirements, and different sources" do
       build_lib "my-gem", path: bundled_app do |s|
         s.add_development_dependency "activesupport"
diff --git a/test/rubygems/test_gem_command_manager.rb b/test/rubygems/test_gem_command_manager.rb
index e1c3512b6f9d04..4c31eb3bc136fa 100644
--- a/test/rubygems/test_gem_command_manager.rb
+++ b/test/rubygems/test_gem_command_manager.rb
@@ -78,7 +78,7 @@ def test_find_command_unknown_suggestions
 
     message = "Unknown command pish".dup
 
-    if defined?(DidYouMean::SPELL_CHECKERS) && defined?(DidYouMean::Correctable)
+    if defined?(DidYouMean)
       message << "\nDid you mean?  \"push\""
     end
 
diff --git a/test/rubygems/test_gem_commands_sources_command.rb b/test/rubygems/test_gem_commands_sources_command.rb
index e004123c905ced..00eb9239940e81 100644
--- a/test/rubygems/test_gem_commands_sources_command.rb
+++ b/test/rubygems/test_gem_commands_sources_command.rb
@@ -60,6 +60,25 @@ def test_execute_add
     assert_equal "", @ui.error
   end
 
+  def test_execute_append
+    setup_fake_source(@new_repo)
+
+    @cmd.handle_options %W[--append #{@new_repo}]
+
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    assert_equal [@gem_repo, @new_repo], Gem.sources
+
+    expected = <<-EOF
+#{@new_repo} added to sources
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
   def test_execute_add_allow_typo_squatting_source
     rubygems_org = "https://rubyems.org"
 
@@ -82,6 +101,28 @@ def test_execute_add_allow_typo_squatting_source
     assert_empty ui.error
   end
 
+  def test_execute_append_allow_typo_squatting_source
+    rubygems_org = "https://rubyems.org"
+
+    setup_fake_source(rubygems_org)
+
+    @cmd.handle_options %W[--append #{rubygems_org}]
+    ui = Gem::MockGemUi.new("y")
+
+    use_ui ui do
+      @cmd.execute
+    end
+
+    expected = "https://rubyems.org is too similar to https://rubygems.org\n\nDo you want to add this source? [yn]  https://rubyems.org added to sources\n"
+
+    assert_equal expected, ui.output
+
+    source = Gem::Source.new(rubygems_org)
+    assert Gem.sources.include?(source)
+
+    assert_empty ui.error
+  end
+
   def test_execute_add_allow_typo_squatting_source_forced
     rubygems_org = "https://rubyems.org"
 
@@ -100,6 +141,24 @@ def test_execute_add_allow_typo_squatting_source_forced
     assert_empty ui.error
   end
 
+  def test_execute_append_allow_typo_squatting_source_forced
+    rubygems_org = "https://rubyems.org"
+
+    setup_fake_source(rubygems_org)
+
+    @cmd.handle_options %W[--force --append #{rubygems_org}]
+
+    @cmd.execute
+
+    expected = "https://rubyems.org added to sources\n"
+    assert_equal expected, ui.output
+
+    source = Gem::Source.new(rubygems_org)
+    assert Gem.sources.include?(source)
+
+    assert_empty ui.error
+  end
+
   def test_execute_add_deny_typo_squatting_source
     rubygems_org = "https://rubyems.org"
 
@@ -125,6 +184,31 @@ def test_execute_add_deny_typo_squatting_source
     assert_empty ui.error
   end
 
+  def test_execute_append_deny_typo_squatting_source
+    rubygems_org = "https://rubyems.org"
+
+    setup_fake_source(rubygems_org)
+
+    @cmd.handle_options %W[--append #{rubygems_org}]
+
+    ui = Gem::MockGemUi.new("n")
+
+    use_ui ui do
+      assert_raise Gem::MockGemUi::TermError do
+        @cmd.execute
+      end
+    end
+
+    expected = "https://rubyems.org is too similar to https://rubygems.org\n\nDo you want to add this source? [yn]  "
+
+    assert_equal expected, ui.output
+
+    source = Gem::Source.new(rubygems_org)
+    refute Gem.sources.include?(source)
+
+    assert_empty ui.error
+  end
+
   def test_execute_add_nonexistent_source
     spec_fetcher
 
@@ -150,6 +234,31 @@ def test_execute_add_nonexistent_source
     assert_equal "", @ui.error
   end
 
+  def test_execute_append_nonexistent_source
+    spec_fetcher
+
+    uri = "http://beta-gems.example.com/specs.#{@marshal_version}.gz"
+    @fetcher.data[uri] = proc do
+      raise Gem::RemoteFetcher::FetchError.new("it died", uri)
+    end
+
+    @cmd.handle_options %w[--append http://beta-gems.example.com]
+
+    use_ui @ui do
+      assert_raise Gem::MockGemUi::TermError do
+        @cmd.execute
+      end
+    end
+
+    expected = <<-EOF
+Error fetching http://beta-gems.example.com:
+\tit died (#{uri})
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
   def test_execute_add_existent_source_invalid_uri
     spec_fetcher
 
@@ -175,6 +284,31 @@ def test_execute_add_existent_source_invalid_uri
     assert_equal "", @ui.error
   end
 
+  def test_execute_append_existent_source_invalid_uri
+    spec_fetcher
+
+    uri = "https://u:p@example.com/specs.#{@marshal_version}.gz"
+
+    @cmd.handle_options %w[--append https://u:p@example.com]
+    @fetcher.data[uri] = proc do
+      raise Gem::RemoteFetcher::FetchError.new("it died", uri)
+    end
+
+    use_ui @ui do
+      assert_raise Gem::MockGemUi::TermError do
+        @cmd.execute
+      end
+    end
+
+    expected = <<-EOF
+Error fetching https://u:REDACTED@example.com:
+\tit died (https://u:REDACTED@example.com/specs.#{@marshal_version}.gz)
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
   def test_execute_add_existent_source_invalid_uri_with_error_by_chance_including_the_uri_password
     spec_fetcher
 
@@ -200,6 +334,31 @@ def test_execute_add_existent_source_invalid_uri_with_error_by_chance_including_
     assert_equal "", @ui.error
   end
 
+  def test_execute_append_existent_source_invalid_uri_with_error_by_chance_including_the_uri_password
+    spec_fetcher
+
+    uri = "https://u:secret@example.com/specs.#{@marshal_version}.gz"
+
+    @cmd.handle_options %w[--append https://u:secret@example.com]
+    @fetcher.data[uri] = proc do
+      raise Gem::RemoteFetcher::FetchError.new("it secretly died", uri)
+    end
+
+    use_ui @ui do
+      assert_raise Gem::MockGemUi::TermError do
+        @cmd.execute
+      end
+    end
+
+    expected = <<-EOF
+Error fetching https://u:REDACTED@example.com:
+\tit secretly died (https://u:REDACTED@example.com/specs.#{@marshal_version}.gz)
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
   def test_execute_add_redundant_source
     spec_fetcher
 
@@ -219,6 +378,25 @@ def test_execute_add_redundant_source
     assert_equal "", @ui.error
   end
 
+  def test_execute_append_redundant_source
+    spec_fetcher
+
+    @cmd.handle_options %W[--append #{@gem_repo}]
+
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    assert_equal [@gem_repo], Gem.sources
+
+    expected = <<-EOF
+#{@gem_repo} moved to end of sources
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
   def test_execute_add_redundant_source_trailing_slash
     repo_with_slash = "http://sample.repo/"
 
@@ -285,6 +463,30 @@ def test_execute_add_http_rubygems_org
     assert_empty @ui.error
   end
 
+  def test_execute_append_http_rubygems_org
+    http_rubygems_org = "http://rubygems.org/"
+
+    setup_fake_source(http_rubygems_org)
+
+    @cmd.handle_options %W[--append #{http_rubygems_org}]
+
+    ui = Gem::MockGemUi.new "n"
+
+    use_ui ui do
+      assert_raise Gem::MockGemUi::TermError do
+        @cmd.execute
+      end
+    end
+
+    assert_equal [@gem_repo], Gem.sources
+
+    expected = <<-EXPECTED
+    EXPECTED
+
+    assert_equal expected, @ui.output
+    assert_empty @ui.error
+  end
+
   def test_execute_add_http_rubygems_org_forced
     rubygems_org = "http://rubygems.org"
 
@@ -303,6 +505,24 @@ def test_execute_add_http_rubygems_org_forced
     assert_empty ui.error
   end
 
+  def test_execute_append_http_rubygems_org_forced
+    rubygems_org = "http://rubygems.org"
+
+    setup_fake_source(rubygems_org)
+
+    @cmd.handle_options %W[--force --append #{rubygems_org}]
+
+    @cmd.execute
+
+    expected = "http://rubygems.org added to sources\n"
+    assert_equal expected, ui.output
+
+    source = Gem::Source.new(rubygems_org)
+    assert Gem.sources.include?(source)
+
+    assert_empty ui.error
+  end
+
   def test_execute_add_https_rubygems_org
     https_rubygems_org = "https://rubygems.org/"
 
@@ -327,6 +547,30 @@ def test_execute_add_https_rubygems_org
     assert_empty @ui.error
   end
 
+  def test_execute_append_https_rubygems_org
+    https_rubygems_org = "https://rubygems.org/"
+
+    setup_fake_source(https_rubygems_org)
+
+    @cmd.handle_options %W[--append #{https_rubygems_org}]
+
+    ui = Gem::MockGemUi.new "n"
+
+    use_ui ui do
+      assert_raise Gem::MockGemUi::TermError do
+        @cmd.execute
+      end
+    end
+
+    assert_equal [@gem_repo], Gem.sources
+
+    expected = <<-EXPECTED
+    EXPECTED
+
+    assert_equal expected, @ui.output
+    assert_empty @ui.error
+  end
+
   def test_execute_add_bad_uri
     @cmd.handle_options %w[--add beta-gems.example.com]
 
@@ -346,6 +590,25 @@ def test_execute_add_bad_uri
     assert_equal "", @ui.error
   end
 
+  def test_execute_append_bad_uri
+    @cmd.handle_options %w[--append beta-gems.example.com]
+
+    use_ui @ui do
+      assert_raise Gem::MockGemUi::TermError do
+        @cmd.execute
+      end
+    end
+
+    assert_equal [@gem_repo], Gem.sources
+
+    expected = <<-EOF
+beta-gems.example.com is not a URI
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
   def test_execute_clear_all
     @cmd.handle_options %w[--clear-all]
 
@@ -530,6 +793,85 @@ def test_execute_update
     assert_equal "", @ui.error
   end
 
+  def test_execute_prepend_new_source
+    setup_fake_source(@new_repo)
+
+    @cmd.handle_options %W[--prepend #{@new_repo}]
+
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    assert_equal [@new_repo, @gem_repo], Gem.sources
+
+    expected = <<-EOF
+#{@new_repo} added to sources
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
+  def test_execute_prepend_existing_source
+    setup_fake_source(@new_repo)
+
+    # Append the source normally first
+    @cmd.handle_options %W[--append #{@new_repo}]
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    # Initial state: [@gem_repo, @new_repo]
+    assert_equal [@gem_repo, @new_repo], Gem.sources
+
+    # Now prepend the existing source
+    @cmd.handle_options %W[--prepend #{@new_repo}]
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    # Should be moved to front: [@new_repo, @gem_repo]
+    assert_equal [@new_repo, @gem_repo], Gem.sources
+
+    expected = <<-EOF
+#{@new_repo} added to sources
+#{@new_repo} moved to top of sources
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
+  def test_execute_append_existing_source
+    setup_fake_source(@new_repo)
+
+    # Prepend the source first so it's at the beginning
+    @cmd.handle_options %W[--prepend #{@new_repo}]
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    # Initial state: [@new_repo, @gem_repo] (new_repo is first)
+    assert_equal [@new_repo, @gem_repo], Gem.sources
+
+    # Now append the existing source
+    @cmd.handle_options %W[--append #{@new_repo}]
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    # Should be moved to end: [@gem_repo, @new_repo]
+    assert_equal [@gem_repo, @new_repo], Gem.sources
+
+    expected = <<-EOF
+#{@new_repo} added to sources
+#{@new_repo} moved to end of sources
+    EOF
+
+    assert_equal expected, @ui.output
+    assert_equal "", @ui.error
+  end
+
   private
 
   def setup_fake_source(uri)
diff --git a/test/rubygems/test_gem_source_list.rb b/test/rubygems/test_gem_source_list.rb
index 64353f8f90df97..5327b14db8e17a 100644
--- a/test/rubygems/test_gem_source_list.rb
+++ b/test/rubygems/test_gem_source_list.rb
@@ -1,8 +1,7 @@
 # frozen_string_literal: true
 
-require "rubygems"
-require "rubygems/source_list"
 require_relative "helper"
+require "rubygems/source_list"
 
 class TestGemSourceList < Gem::TestCase
   def setup
@@ -116,4 +115,128 @@ def test_delete_a_source
     @sl.delete Gem::Source.new(@uri)
     assert_equal @sl.sources, []
   end
+
+  def test_prepend_new_source
+    uri2 = "http://example2"
+    source2 = Gem::Source.new(uri2)
+
+    result = @sl.prepend(uri2)
+
+    assert_kind_of Gem::Source, result
+    assert_kind_of Gem::URI, result.uri
+    assert_equal uri2, result.uri.to_s
+    assert_equal [source2, @source], @sl.sources
+  end
+
+  def test_prepend_existing_source
+    uri2 = "http://example2"
+    source2 = Gem::Source.new(uri2)
+    @sl << uri2
+
+    assert_equal [@source, source2], @sl.sources
+
+    result = @sl.prepend(uri2)
+
+    assert_kind_of Gem::Source, result
+    assert_kind_of Gem::URI, result.uri
+    assert_equal uri2, result.uri.to_s
+    assert_equal [source2, @source], @sl.sources
+  end
+
+  def test_prepend_alias_behaves_like_unshift
+    sl = Gem::SourceList.new
+
+    uri1 = "http://one"
+    uri2 = "http://two"
+
+    source1 = sl << uri1
+    source2 = sl << uri2
+
+    # move existing to front
+    result = sl.prepend(uri2)
+
+    assert_kind_of Gem::Source, result
+    assert_equal [source2, source1], sl.sources
+
+    # and again with the other
+    result = sl.prepend(uri1)
+    assert_equal [source1, source2], sl.sources
+  end
+
+  def test_append_method_new_source
+    sl = Gem::SourceList.new
+
+    uri1 = "http://example1"
+
+    result = sl.append(uri1)
+
+    assert_kind_of Gem::Source, result
+    assert_kind_of Gem::URI, result.uri
+    assert_equal uri1, result.uri.to_s
+    assert_equal [result], sl.sources
+  end
+
+  def test_append_method_existing_moves_to_end
+    sl = Gem::SourceList.new
+
+    uri1 = "http://example1"
+    uri2 = "http://example2"
+
+    s1 = sl << uri1
+    s2 = sl << uri2
+
+    # list is [s1, s2]; appending s1 should move it to end => [s2, s1]
+    result = sl.append(uri1)
+
+    assert_equal s1, result
+    assert_equal [s2, s1], sl.sources
+  end
+
+  def test_prepend_with_gem_source_object
+    sl = Gem::SourceList.new
+
+    uri1 = "http://example1"
+    uri2 = "http://example2"
+    source1 = Gem::Source.new(uri1)
+    source2 = Gem::Source.new(uri2)
+
+    # Add first source
+    sl << source1
+
+    # Prepend with Gem::Source object
+    result = sl.prepend(source2)
+
+    assert_equal source2, result
+    assert_equal [source2, source1], sl.sources
+
+    # Prepend existing source - should move to front
+    result = sl.prepend(source1)
+
+    assert_equal source1, result
+    assert_equal [source1, source2], sl.sources
+  end
+
+  def test_append_with_gem_source_object
+    sl = Gem::SourceList.new
+
+    uri1 = "http://example1"
+    uri2 = "http://example2"
+    source1 = Gem::Source.new(uri1)
+    source2 = Gem::Source.new(uri2)
+
+    # Add first source
+    sl << source1
+
+    # Append with Gem::Source object
+    result = sl.append(source2)
+
+    assert_equal source2, result
+    assert_equal [source1, source2], sl.sources
+
+    # Append existing source - should move to end
+    result = sl.append(source1)
+
+    assert_equal source1, result
+    assert_equal [source2, source1], sl.sources
+  end
 end