BUG: Raise clear error for duplicate id_vars in melt (GH61475) (#61484)

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

* Bug Fixed Issue GH61475

* if statement changes

* Update pandas/core/reshape/melt.py

---------

Co-authored-by: ZanirP <zanir@DESKTOP-JOHE35M>
Co-authored-by: ZanirP <[email protected]>
Co-authored-by: Matthew Roeschke <[email protected]>

Author: 4 people

doc/source/whatsnew/v3.0.0.rst

@@ -851,6 +851,7 @@ 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

@@ -182,6 +182,10 @@ def melt(
     value_vars_was_not_none = value_vars is not None
     value_vars = ensure_list_vars(value_vars, "value_vars", frame.columns)
 
+    # GH61475 - prevent AttributeError when duplicate column in id_vars
+    if len(frame.columns.get_indexer_for(id_vars)) > len(id_vars):
+        raise ValueError("id_vars cannot contain duplicate columns.")
+
     if id_vars or value_vars:
         if col_level is not None:
             level = frame.columns.get_level_values(col_level)

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 = "id_vars cannot contain duplicate columns."
+
+        with pytest.raises(ValueError, match=msg):
+            df.melt(id_vars=["A"], value_vars=["B"])
+
 
 class TestLreshape:
     def test_pairs(self):