Pandas BUG: Duplicate columns get duplicated in DataFrame.align

# 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.

Comment From: asadullahnaeem

# 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 
# 0  2  3 
# 1  5  6