API: Add NDFrame property to disallow duplicates
edit: see https://github.com/pandas-dev/pandas/issues/27108#issuecomment-527633246

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I'd like to be able to have an index, and ensure that no operation introduces duplicates.

```python
idx = pd.Index(..., allow_duplicates=False)
s = pd.Series(..., index=idx)
```

From here, any pandas operation that introduces duplicates (e.g. `s.loc[['a', 'a']]`) would raise, rather than return an Index with two values.