read_excel() modifies provided types dict when accessing file with duplicate column

[cdol]

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of pandas.
• (optional) I have confirmed this bug exists on the master branch of pandas.

test.xlsx :

a	a	b	c
1	1	b1	c1
2	2	b2	c2
3	3	b3	c3

import pandas as pd


types_dict = {'a': str,
             'b': str,
             'c': str,
             }


if __name__ == "__main__":
    df = pd.read_excel('./test.xlsx', dtype=type_dict)
    print(list(type_dict.keys()))

>> ['a', 'b', 'c', 'a.1']

Bug/Issue description:
When using dtype loading a .xlsx-file with a duplicate column into a dataframe modifies the provided types_dict / adds entries for duplicate columns.

It seems to me like the modification of the types_dict is an unwanted side effect.

[mzeitlin11]

Thanks for reporting this @cdol! Confirmed on master (seems limited to read_excel, doesn't show up for similar example with read_csv), agree it is an unwelcome side effect. Fixes very welcome!