Added RuntimeWarning to lib.fast_unique_multiple
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Looks like at ```L340```, it triggers the warning, but only at the AP ```Linux py37_np_dev```

https://github.com/pandas-dev/pandas/blob/42ef40971261b286b53ddfc85d2e399067b7c72d/pandas/tests/indexes/multi/test_setops.py#L334-L345

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@jbrockmendel Does having something like this:

```python
from pandas.compat.numpy import _is_numpy_dev

...

if _is_numpy_dev:
    with tm.assert_produces_warning(RuntimeWarning):
        result = idx.union(idx[:1]) 
else:
    result = idx.union(idx[:1]) 
```

is the correct approach? Usually we would write

```
warn = RuntimeWarning if _is_numpy_dev else None
with tm.assert_produces_warning(warn):
    ...
``` So apparently I never needed the ```_is_numpy_dev```. \*facepalm\* ping @jreback  thanks @MomIsBestFriend 