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

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [X] I have confirmed this bug exists on the master branch of pandas.


### Reproducible Example

```python
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

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

</details>
