I know I can find duplicate columns using:

```
df.T.duplicated()

```

what I'd like to know the index that a duplicate column is a duplicate of. For example, both `C` and `D` are duplicates of a `A` below:

```
df = pd.DataFrame([[1,0,1,1], [2,0,2,2]], columns=['A', 'B', 'C', 'D'])

   A  B  C  D
0  1  0  1  1
1  2  0  2  2

```

I'd like something like:

```
duplicate_index = pd.Series([None, None, 'A', 'A'], ['A', 'B', 'C', 'D'])

```