[BUG] DataFrame loc indexing is incorrect with repeated column labels.

[wence-]

Describe the bug

This is basically #13266 but for loc, I will fix it separately due to different code paths.

import cudf
import pandas as pd
import numpy as np
df = pd.DataFrame(np.arange(4).reshape(2, 2))
cdf = cudf.from_pandas(df)

df.loc[:, [0, 1, 0]]
#    0  1  0
# 0  0  1  0
# 1  2  3  2

cdf.loc[:, [0, 1, 0]]
#    0  1
# 0  0  1
# 1  2  3

This is because ColumnAccessor.select_by_label uniquifies input label arguments.

Expected behavior

This should match pandas.

[wence-]

This is a consequence of #13273, and how it will be fixed depends on what we do there.

[mroeschke]

This was fixed by #16514 so closing