CLN: improve is_unique check in Index.intersection
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

```
       before           after         ratio
     [e99e5ab3]       [79d6a50f]
     <cln_intersection~1>       <cln_intersection>
-        918±20ns          819±2ns     0.89  index_object.Range.time_min_trivial
-         465±7ns          414±7ns     0.89  index_object.Range.time_get_loc_dec
-         464±6ns          411±7ns     0.89  index_object.Range.time_get_loc_inc
-      16.6±0.7ms      12.6±0.03ms     0.76  index_object.SetOperations.time_operation('date_string', 'union')
```

Seems to have no impact.

cc @jreback
