groupby error when grouping by float index with non-unique values
#### Code Sample, a copy-pastable example if possible

```python
pd.Series([2, 5, 6, 8], [2.0, 4.0, 4.0, 5.0]).groupby(level=0)

```
#### Problem description

This should just work instead of raising.
#### Expected Output

#### Output of ``pd.show_versions()``

<details>

pandas           : 0.26.0.dev0+593.g9d45934af
numpy            : 1.17.0

</details>
