BUG: concat raises when all indexes are the same and keys are duplicated

[rhshadrach]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the master branch of pandas.

Reproducible Example

keys = ["e", "e"]
df = pd.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
result = pd.concat([df, df], axis=1, keys=keys)

Issue Description

concat raises when all the indexes of objs argument are the same and there are duplicates in the keys argument.

Expected Behavior

Not to raise.

Installed Versions

Replace this line with the output of pd.show_versions()