BUG: Error raised accessing tuple value in Series when index contains duplicates
E.g.

```
s = pd.Series([1, 1], index=[(1, 1), (1, 1)])
s[(1, 1)]
```

raises `KeyError: (1, 1)`. Within `pandas._libs.index.IndexEngine._get_loc_duplicates`, we're using ndarray.searchsorted which interprets the tuple as an array-like of values to search for rather than a single tuple.