BUG: Raise clear error for duplicate id_vars in melt (GH61475)

Author: ZanirP

doc/source/whatsnew/v3.0.0.rst

@@ -847,6 +847,7 @@ Reshaping
 - Bug in :meth:`DataFrame.stack` with the new implementation where ``ValueError`` is raised when ``level=[]`` (:issue:`60740`)
 - Bug in :meth:`DataFrame.unstack` producing incorrect results when manipulating empty :class:`DataFrame` with an :class:`ExtentionDtype` (:issue:`59123`)
 - Bug in :meth:`concat` where concatenating DataFrame and Series with ``ignore_index = True`` drops the series name (:issue:`60723`, :issue:`56257`)
+- Bug in :func:`melt` where calling with duplicate column names in ``id_vars`` raised a misleading ``AttributeError`` (:issue:`61475`)
 
 Sparse
 ^^^^^^

pandas/core/reshape/melt.py

@@ -239,6 +239,9 @@ 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

@@ -555,6 +555,14 @@ def test_melt_multiindex_columns_var_name_too_many(self):
         ):
             df.melt(var_name=["first", "second", "third"])
 
+    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"
+
+        with pytest.raises(Exception, match=msg):
+            df.melt(id_vars=["A"], value_vars=["B"])
+
 
 class TestLreshape:
     def test_pairs(self):