Support duplicated columns (maybe?)

[phofl]

We currently don't support duplicated columns, introducing support is not completely trivial since we would have to handle them properly in column projections. One example to explain the difficulties:

pdf = pd.DataFrame([[1, 2], [3, 4]], columns=list("aa"))
print(pdf["a"])

print(pdf[["a", "a"]])

0  1  2
1  3  4
   a  a  a  a
0  1  2  1  2
1  3  4  3  4

Selecting duplicated columns once returns all of them, but having them n times in the indexer duplicates the columns also n times, so we would have to be very careful to keep the count consistent. Other things also depend on set comparisons, which makes this even harder.

I don#t think that this is a very important feature for now, so I don't intend to prioritise this anytime soon