Flatten a single-leveled multiIndex

问题

Indexing into a Pandas DataFrame throws an error when:

• The query has multiple copies of the same value.
• The index in the DataFrame is a single-level MultiIndex.

The below code shows a simple example

import pandas as pd
# columns as str class -- works
D = pd.DataFrame([[1,2]], columns = ['A','B'], index = ['R1'])
print(D.loc[:,['A','A']], '\n')   # OK

# columns as Index class -- works
D = pd.DataFrame([[1,2]], columns = pd.Index(['A','B']), index = ['R1'])
print(D.loc[:,['A','A']], '\n')  # OK

# columns as single-level MultiIndex -- fails
D = pd.DataFrame([[1,2]], columns = pd.MultiIndex.from_arrays([['A','B']]), index = ['R1'])
print(D.loc[:,['A','A']])   # error "Index._join_level on non-unique index is not implemented"

In the last case, D.columns is

MultiIndex([('A',),
            ('B',)],
           )

Why does this occur? What is the easiest way to fix it?

In my program, I do not construct the DataFrame columns directly (as I do in the simple example above); instead, it is obtained via a drop_level command (producing the single-level MultiIndex). So, simply creating a DataFrame using Index class is not an option.

来源：https://stackoverflow.com/questions/65379153/flatten-a-single-leveled-multiindex