如何将熊猫DataFrame转换为以下显示？(从水平到垂直；带重复键)

import pandas as pd

df = pd.DataFrame({'A': ['foo', 'foo', 'foo', 'foo', 'foo', 'bar', 'bar', 'bar', 'bar'],
                   'B': ['one', 'one', 'one', 'two', 'two', 'one', 'one', 'two', 'two'],
                   'C': ['x', 'y', 'x', 'y', 'x', 'y', 'x', 'y', 'x'],
                   'D': [1, 3, 2, 5, 4, 1, 2, 4, 3]})

melted_df = pd.melt(df, id_vars=['A', 'B'], value_vars=['C', 'D'])

print(melted_df)

    A    B variable  value
0  foo  one        C      x
1  foo  one        C      y
2  foo  one        C      x
3  foo  two        C      y
4  foo  two        C      x
5  bar  one        C      y
6  bar  one        C      x
7  bar  two        C      y
8  bar  two        C      x
9  foo  one        D      1
10 foo  one        D      3
11 foo  one        D      2
12 foo  two        D      5
13 foo  two        D      4
14 bar  one        D      1
15 bar  one        D      2
16 bar  two        D      4
17 bar  two        D      3