diff --git a/.gitignore b/.gitignore
index 3e8d3310f555b4..ddf8e9a99adae3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -21,6 +21,7 @@
 *.pch
 *.pdb
 *.rbinc
+*.rbbin
 *.rej
 *.s
 *.sav
diff --git a/ext/json/lib/json/common.rb b/ext/json/lib/json/common.rb
index 486ec62a582450..9a878cead9f6a3 100644
--- a/ext/json/lib/json/common.rb
+++ b/ext/json/lib/json/common.rb
@@ -48,7 +48,7 @@ def array_class_proc(array_class, on_load)
         end
       end
 
-      # TODO: exctract :create_additions support to another gem for version 3.0
+      # TODO: extract :create_additions support to another gem for version 3.0
       def create_additions_proc(opts)
         if opts[:symbolize_names]
           raise ArgumentError, "options :symbolize_names and :create_additions cannot be  used in conjunction"
@@ -87,31 +87,32 @@ def create_additions_proc(opts)
         opts
       end
 
-      GEM_ROOT = File.expand_path("../../../", __FILE__) + "/"
       def create_additions_warning
-        message = "JSON.load implicit support for `create_additions: true` is deprecated " \
+        JSON.deprecation_warning "JSON.load implicit support for `create_additions: true` is deprecated " \
           "and will be removed in 3.0, use JSON.unsafe_load or explicitly " \
           "pass `create_additions: true`"
+      end
+    end
+  end
 
-        uplevel = 4
-        caller_locations(uplevel, 10).each do |frame|
-          if frame.path.nil? || frame.path.start_with?(GEM_ROOT) || frame.path.end_with?("/truffle/cext_ruby.rb", ".c")
-            uplevel += 1
-          else
-            break
-          end
-        end
-
-        if RUBY_VERSION >= "3.0"
-          warn(message, uplevel: uplevel - 1, category: :deprecated)
+  class << self
+    def deprecation_warning(message, uplevel = 3) # :nodoc:
+      gem_root = File.expand_path("../../../", __FILE__) + "/"
+      caller_locations(uplevel, 10).each do |frame|
+        if frame.path.nil? || frame.path.start_with?(gem_root) || frame.path.end_with?("/truffle/cext_ruby.rb", ".c")
+          uplevel += 1
         else
-          warn(message, uplevel: uplevel - 1)
+          break
         end
       end
+
+      if RUBY_VERSION >= "3.0"
+        warn(message, uplevel: uplevel, category: :deprecated)
+      else
+        warn(message, uplevel: uplevel)
+      end
     end
-  end
 
-  class << self
     # :call-seq:
     #   JSON[object] -> new_array or new_string
     #
diff --git a/ext/json/parser/parser.c b/ext/json/parser/parser.c
index 01b6e6293b7aaf..ab9d6c205e69b2 100644
--- a/ext/json/parser/parser.c
+++ b/ext/json/parser/parser.c
@@ -422,10 +422,12 @@ static void emit_parse_warning(const char *message, JSON_ParserState *state)
     long line, column;
     cursor_position(state, &line, &column);
 
-    rb_warn("%s at line %ld column %ld", message, line, column);
+    VALUE warning = rb_sprintf("%s at line %ld column %ld", message, line, column);
+    rb_funcall(mJSON, rb_intern("deprecation_warning"), 1, warning);
 }
 
 #define PARSE_ERROR_FRAGMENT_LEN 32
+
 #ifdef RBIMPL_ATTR_NORETURN
 RBIMPL_ATTR_NORETURN()
 #endif
@@ -830,21 +832,64 @@ static inline VALUE json_decode_array(JSON_ParserState *state, JSON_ParserConfig
     return array;
 }
 
+static VALUE json_find_duplicated_key(size_t count, const VALUE *pairs)
+{
+    VALUE set = rb_hash_new_capa(count / 2);
+    for (size_t index = 0; index < count; index += 2) {
+        size_t before = RHASH_SIZE(set);
+        VALUE key = pairs[index];
+        rb_hash_aset(set, key, Qtrue);
+        if (RHASH_SIZE(set) == before) {
+            if (RB_SYMBOL_P(key)) {
+                return rb_sym2str(key);
+            }
+            return key;
+        }
+    }
+    return Qfalse;
+}
+
+static void emit_duplicate_key_warning(JSON_ParserState *state, VALUE duplicate_key)
+{
+    VALUE message = rb_sprintf(
+        "detected duplicate key %"PRIsVALUE" in JSON object. This will raise an error in json 3.0 unless enabled via `allow_duplicate_key: true`",
+        rb_inspect(duplicate_key)
+    );
+
+    emit_parse_warning(RSTRING_PTR(message), state);
+    RB_GC_GUARD(message);
+}
+
+#ifdef RBIMPL_ATTR_NORETURN
+RBIMPL_ATTR_NORETURN()
+#endif
+static void raise_duplicate_key_error(JSON_ParserState *state, VALUE duplicate_key)
+{
+    VALUE message = rb_sprintf(
+        "duplicate key %"PRIsVALUE,
+        rb_inspect(duplicate_key)
+    );
+
+    raise_parse_error(RSTRING_PTR(message), state);
+    RB_GC_GUARD(message);
+}
+
 static inline VALUE json_decode_object(JSON_ParserState *state, JSON_ParserConfig *config, size_t count)
 {
     size_t entries_count = count / 2;
     VALUE object = rb_hash_new_capa(entries_count);
-    rb_hash_bulk_insert(count, rvalue_stack_peek(state->stack, count), object);
+    const VALUE *pairs = rvalue_stack_peek(state->stack, count);
+    rb_hash_bulk_insert(count, pairs, object);
 
     if (RB_UNLIKELY(RHASH_SIZE(object) < entries_count)) {
         switch (config->on_duplicate_key) {
             case JSON_IGNORE:
                 break;
             case JSON_DEPRECATED:
-                emit_parse_warning("detected duplicate keys in JSON object. This will raise an error in json 3.0 unless enabled via `allow_duplicate_key: true`", state);
+                emit_duplicate_key_warning(state, json_find_duplicated_key(count, pairs));
                 break;
             case JSON_RAISE:
-                raise_parse_error("duplicate key", state);
+                raise_duplicate_key_error(state, json_find_duplicated_key(count, pairs));
                 break;
         }
     }
diff --git a/lib/bundler/cli/common.rb b/lib/bundler/cli/common.rb
index be21ba8299370f..7608adf2ef9e9c 100644
--- a/lib/bundler/cli/common.rb
+++ b/lib/bundler/cli/common.rb
@@ -130,7 +130,7 @@ def self.patch_level_options(options)
     def self.clean_after_install?
       clean = Bundler.settings[:clean]
       return clean unless clean.nil?
-      clean ||= Bundler.feature_flag.bundler_4_mode? && Bundler.settings[:path].nil?
+      clean ||= Bundler.feature_flag.bundler_5_mode? && Bundler.settings[:path].nil?
       clean &&= !Bundler.use_system_gems?
       clean
     end
diff --git a/lib/bundler/feature_flag.rb b/lib/bundler/feature_flag.rb
index a8fa2a1bdee04a..2cf58eefd60f27 100644
--- a/lib/bundler/feature_flag.rb
+++ b/lib/bundler/feature_flag.rb
@@ -30,7 +30,7 @@ def self.settings_method(name, key, &default)
     settings_flag(:allow_offline_install) { bundler_4_mode? }
     settings_flag(:cache_all) { bundler_4_mode? }
     settings_flag(:forget_cli_options) { bundler_4_mode? }
-    settings_flag(:global_gem_cache) { bundler_4_mode? }
+    settings_flag(:global_gem_cache) { bundler_5_mode? }
     settings_flag(:lockfile_checksums) { bundler_4_mode? }
     settings_flag(:plugins) { @bundler_version >= Gem::Version.new("1.14") }
     settings_flag(:update_requires_all_flag) { bundler_5_mode? }
diff --git a/lib/bundler/settings.rb b/lib/bundler/settings.rb
index d036754a78371a..cc20598fd9c2c2 100644
--- a/lib/bundler/settings.rb
+++ b/lib/bundler/settings.rb
@@ -272,7 +272,7 @@ def path
       def use_system_gems?
         return true if system_path
         return false if explicit_path
-        !Bundler.feature_flag.bundler_4_mode?
+        !Bundler.feature_flag.bundler_5_mode?
       end
 
       def base_path
diff --git a/spec/bundler/commands/clean_spec.rb b/spec/bundler/commands/clean_spec.rb
index 99a8f9c1416e5f..fd5a0fdbcaa2ff 100644
--- a/spec/bundler/commands/clean_spec.rb
+++ b/spec/bundler/commands/clean_spec.rb
@@ -151,7 +151,7 @@ def should_not_have_gems(*gems)
     bundle :clean
 
     digest = Digest(:SHA1).hexdigest(git_path.to_s)
-    cache_path = Bundler.feature_flag.global_gem_cache? ? home(".bundle/cache/git/foo-1.0-#{digest}") : vendored_gems("cache/bundler/git/foo-1.0-#{digest}")
+    cache_path = vendored_gems("cache/bundler/git/foo-1.0-#{digest}")
     expect(cache_path).to exist
   end
 
@@ -427,7 +427,7 @@ def should_not_have_gems(*gems)
     should_not_have_gems "foo-1.0"
   end
 
-  it "automatically cleans when path has not been set", bundler: "4" do
+  it "automatically cleans when path has not been set", bundler: "5" do
     build_repo2
 
     install_gemfile <<-G
diff --git a/spec/bundler/commands/install_spec.rb b/spec/bundler/commands/install_spec.rb
index 22eb64ca81a0e0..e7c1f5e456e2d2 100644
--- a/spec/bundler/commands/install_spec.rb
+++ b/spec/bundler/commands/install_spec.rb
@@ -30,22 +30,29 @@
     end
 
     it "does not create ./.bundle by default" do
-      gemfile <<-G
+      install_gemfile <<-G
         source "https://gem.repo1"
         gem "myrack"
       G
 
-      bundle :install # can't use install_gemfile since it sets retry
       expect(bundled_app(".bundle")).not_to exist
     end
 
+    it "will create a ./.bundle by default", bundler: "5" do
+      install_gemfile <<-G
+        source "https://gem.repo1"
+        gem "myrack"
+      G
+
+      expect(bundled_app(".bundle")).to exist
+    end
+
     it "does not create ./.bundle by default when installing to system gems" do
-      gemfile <<-G
+      install_gemfile <<-G, env: { "BUNDLE_PATH__SYSTEM" => "true" }
         source "https://gem.repo1"
         gem "myrack"
       G
 
-      bundle :install, env: { "BUNDLE_PATH__SYSTEM" => "true" } # can't use install_gemfile since it sets retry
       expect(bundled_app(".bundle")).not_to exist
     end
 
diff --git a/spec/bundler/install/gems/compact_index_spec.rb b/spec/bundler/install/gems/compact_index_spec.rb
index 823b5aab110b03..e5f7380855befd 100644
--- a/spec/bundler/install/gems/compact_index_spec.rb
+++ b/spec/bundler/install/gems/compact_index_spec.rb
@@ -1010,11 +1010,7 @@ def start
         gem "myrack"
       G
 
-      gem_path = if Bundler.feature_flag.global_gem_cache?
-        default_cache_path.dirname.join("cache", "gems", "localgemserver.test.80.dd34752a738ee965a2a4298dc16db6c5", "myrack-1.0.0.gem")
-      else
-        default_cache_path.dirname.join("myrack-1.0.0.gem")
-      end
+      gem_path = default_cache_path.dirname.join("myrack-1.0.0.gem")
 
       expect(exitstatus).to eq(37)
       expect(err).to eq <<~E.strip
diff --git a/spec/bundler/support/filters.rb b/spec/bundler/support/filters.rb
index 15b4adf62e387b..6e94b10e32c845 100644
--- a/spec/bundler/support/filters.rb
+++ b/spec/bundler/support/filters.rb
@@ -32,7 +32,7 @@ def inspect
 
   config.filter_run_when_matching :focus unless ENV["CI"]
 
-  config.before(:each, bundler: "4") do
-    bundle "config simulate_version 4"
+  config.before(:each, :bundler) do |example|
+    bundle "config simulate_version #{example.metadata[:bundler]}"
   end
 end
diff --git a/spec/bundler/support/path.rb b/spec/bundler/support/path.rb
index 7c2d06abaf55b8..cc750f55d81f53 100644
--- a/spec/bundler/support/path.rb
+++ b/spec/bundler/support/path.rb
@@ -136,19 +136,11 @@ def home(*path)
     end
 
     def default_bundle_path(*path)
-      if Bundler.feature_flag.bundler_4_mode?
-        local_gem_path(*path)
-      else
-        system_gem_path(*path)
-      end
+      system_gem_path(*path)
     end
 
     def default_cache_path(*path)
-      if Bundler.feature_flag.global_gem_cache?
-        home(".bundle/cache", *path)
-      else
-        default_bundle_path("cache/bundler", *path)
-      end
+      default_bundle_path("cache/bundler", *path)
     end
 
     def compact_index_cache_path
diff --git a/test/json/json_parser_test.rb b/test/json/json_parser_test.rb
index 106492e1c41a9c..6455d2971ae3c0 100644
--- a/test/json/json_parser_test.rb
+++ b/test/json/json_parser_test.rb
@@ -333,11 +333,36 @@ def test_parse_big_integers
 
   def test_parse_duplicate_key
     expected = {"a" => 2}
+    expected_sym = {a: 2}
+
     assert_equal expected, parse('{"a": 1, "a": 2}', allow_duplicate_key: true)
     assert_raise(ParserError) { parse('{"a": 1, "a": 2}', allow_duplicate_key: false) }
-    assert_deprecated_warning(/duplicate keys/) do
+    assert_raise(ParserError) { parse('{"a": 1, "a": 2}', allow_duplicate_key: false, symbolize_names: true) }
+
+    assert_deprecated_warning(/duplicate key "a"/) do
       assert_equal expected, parse('{"a": 1, "a": 2}')
     end
+    assert_deprecated_warning(/duplicate key "a"/) do
+      assert_equal expected_sym, parse('{"a": 1, "a": 2}', symbolize_names: true)
+    end
+
+    if RUBY_ENGINE == 'RUBY_ENGINE'
+      assert_deprecated_warning(/#{File.basename(__FILE__)}\:#{__LINE__ + 1}/) do
+        assert_equal expected, parse('{"a": 1, "a": 2}')
+      end
+    end
+
+    unless RUBY_ENGINE == 'jruby'
+      assert_raise(ParserError) do
+        fake_key = Object.new
+        JSON.load('{"a": 1, "a": 2}', -> (obj) { obj == "a" ? fake_key : obj }, allow_duplicate_key: false)
+      end
+
+      assert_deprecated_warning(/duplicate key #<Object:0x/) do
+        fake_key = Object.new
+        JSON.load('{"a": 1, "a": 2}', -> (obj) { obj == "a" ? fake_key : obj })
+      end
+    end
   end
 
   def test_some_wrong_inputs