Bug Fixed Issue GH61475

Author: ZanirP

pandas/core/reshape/melt.py

@@ -173,6 +173,9 @@ def melt(
     1      b          B          E      3
     2      c          B          E      5
     """
+    # GH61475 - prevent AttributeError when duplicate column in id_vars
+    if id_vars and any(frame.columns.tolist().count(col) > 1 for col in id_vars):
+        raise ValueError("id_vars cannot contain duplicate columns.")
     if value_name in frame.columns:
         raise ValueError(
             f"value_name ({value_name}) cannot match an element in "
@@ -239,9 +242,6 @@ def melt(
     mdata: dict[Hashable, AnyArrayLike] = {}
     for col in id_vars:
         id_data = frame.pop(col)
-        # GH61475 - prevent AttributeError when duplicate column
-        if not hasattr(id_data, "dtype"):
-            raise Exception(f"{col} is a duplicate column header")
         if not isinstance(id_data.dtype, np.dtype):
             # i.e. ExtensionDtype
             if num_cols_adjusted > 0:

pandas/tests/reshape/test_melt.py

@@ -558,9 +558,9 @@ def test_melt_multiindex_columns_var_name_too_many(self):
     def test_melt_duplicate_column_header_raises(self):
         # GH61475
         df = DataFrame([[1, 2, 3], [3, 4, 5]], columns=["A", "A", "B"])
-        msg = "A is a duplicate column header"
+        msg = "id_vars cannot contain duplicate columns."
 
-        with pytest.raises(Exception, match=msg):
+        with pytest.raises(ValueError, match=msg):
             df.melt(id_vars=["A"], value_vars=["B"])