Improve error message about duplicate columns in df.explode (#49264)

Author: a-mroz

pandas/core/frame.py

@@ -8956,7 +8956,11 @@ def explode(
         3    4  1    e
         """
         if not self.columns.is_unique:
-            raise ValueError("columns must be unique")
+            duplicate_cols = self.columns[self.columns.duplicated()].tolist()
+            raise ValueError(
+                "DataFrame columns must be unique. "
+                + f"Duplicate columns: {duplicate_cols}"
+            )
 
         columns: list[Hashable]
         if is_scalar(column) or isinstance(column, tuple):

pandas/tests/frame/methods/test_explode.py

@@ -1,3 +1,5 @@
+import re
+
 import numpy as np
 import pytest
 
@@ -18,7 +20,10 @@ def test_error():
         df.explode(list("AA"))
 
     df.columns = list("AA")
-    with pytest.raises(ValueError, match="columns must be unique"):
+    with pytest.raises(
+        ValueError,
+        match=re.escape("DataFrame columns must be unique. Duplicate columns: ['A']"),
+    ):
         df.explode("A")