BUG: Duplicate columns get duplicated in DataFrame.align #50845
Description
# With duplicate columns df = pd.DataFrame([[1, 2, 3], [4, 5, 6]], columns=list('abb')) result = df.drop(columns='a').align(df, join="inner", copy=False) print(result[0]) # b b b b # 0 2 2 3 3 # 1 5 5 6 6 # Without duplicate columns df = pd.DataFrame([[1, 2, 3], [4, 5, 6]], columns=list('abc')) result = df.drop(columns='a').align(df, join="inner", copy=False) print(result[0]) # b c # 0 2 3 # 1 5 6 # With duplicate columns but without the drop(columns=...) df = pd.DataFrame([[1, 2, 3], [4, 5, 6]], columns=list('abb')) result = df.align(df, join="inner", copy=False) print(result[0]) # a b b # 0 1 2 3 # 1 4 5 6 Duplicate columns get duplicated again in alignment. If self and other have the same columns, then a different code path is taken and the bug does not appear.