diff --git a/doc/changes/dev/13447.newcontrib.rst b/doc/changes/dev/13447.newcontrib.rst
new file mode 100644
index 00000000000..c1d81c56626
--- /dev/null
+++ b/doc/changes/dev/13447.newcontrib.rst
@@ -0,0 +1 @@
+Improve the error message raised by :func:`mne.viz.plot_topomap` (via :func:`mne.channels.layout._auto_topomap_coords`) when mixed 10–20 EEG electrode naming conventions are detected, by :newcontrib:`Aasma Gupta`.
\ No newline at end of file
diff --git a/doc/changes/names.inc b/doc/changes/names.inc
index b684d067a1d..33df89820ae 100644
--- a/doc/changes/names.inc
+++ b/doc/changes/names.inc
@@ -1,4 +1,5 @@
 .. _Aaron Earle-Richardson: https://github.com/Aaronearlerichardson
+.. _Aasma Gupta: https://github.com/AasmaGupta
 .. _Abram Hindle: https://softwareprocess.es
 .. _Adam Li: https://github.com/adam2392
 .. _Adeline Fecker: https://github.com/adelinefecker
diff --git a/mne/channels/layout.py b/mne/channels/layout.py
index 85b7913c1c6..fe7a1026a8e 100644
--- a/mne/channels/layout.py
+++ b/mne/channels/layout.py
@@ -971,11 +971,36 @@ def _auto_topomap_coords(info, picks, ignore_overlap, to_sphere, sphere):
             for elec_i in squareform(dist < 1e-10).any(axis=0).nonzero()[0]
         ]
 
-        raise ValueError(
-            "The following electrodes have overlapping positions,"
-            " which causes problems during visualization:\n"
-            + ", ".join(problematic_electrodes)
-        )
+        duplicates_1020 = {
+            "T3": "T7",
+            "T4": "T8",
+            "T5": "P7",
+            "T6": "P8",
+        }
+
+        names = set(problematic_electrodes)
+        duplicate_pairs = [
+            (old, new)
+            for old, new in duplicates_1020.items()
+            if old in names and new in names
+        ]
+
+        if duplicate_pairs:
+            raise ValueError(
+                "Duplicate EEG electrode positions detected due to mixed 10-20 "
+                "naming conventions.\n"
+                "You appear to have both legacy (T3/T4/T5/T6) and modern "
+                "(T7/T8/P7/P8) electrode names present. The modern convention "
+                "(T7/T8/P7/P8) is recommended.\n\n"
+                "Please drop the legacy channels before plotting, for example:\n"
+                "    inst.drop_channels(['T3', 'T4', 'T5', 'T6'])"
+            )
+
+        else:
+            raise ValueError(
+                "The following electrodes have overlapping positions, which causes "
+                "problems during visualization:\n" + ", ".join(problematic_electrodes)
+            )
 
     if to_sphere:
         # translate to sphere origin, transform/flatten Z, translate back
diff --git a/mne/channels/tests/test_layout.py b/mne/channels/tests/test_layout.py
index ad44a6873a4..7a7c0358b31 100644
--- a/mne/channels/tests/test_layout.py
+++ b/mne/channels/tests/test_layout.py
@@ -15,6 +15,7 @@
     assert_equal,
 )
 
+import mne
 from mne import pick_info, pick_types
 from mne._fiff.constants import FIFF
 from mne._fiff.meas_info import _empty_info
@@ -186,6 +187,24 @@ def test_find_topomap_coords():
         _find_topomap_coords(info, picks, **kwargs)
 
 
+def test_duplicate_1020_electrode_names_error():
+    """Ensure mixed 10–20 EEG naming conventions raise a clear error."""
+    montage = mne.channels.make_standard_montage("standard_1020")
+    data = np.random.randn(len(montage.ch_names))
+    info = mne.create_info(
+        ch_names=montage.ch_names,
+        sfreq=1000,
+        ch_types="eeg",
+    )
+    info.set_montage(montage)
+
+    with pytest.raises(
+        ValueError,
+        match="Duplicate EEG electrode positions detected due to mixed 10-20",
+    ):
+        mne.viz.plot_topomap(data, pos=info)
+
+
 def test_make_eeg_layout(tmp_path):
     """Test creation of EEG layout."""
     lout_orig = read_layout(fname=lout_path / "test_raw.lout")