BUG: declare and use self.unique_check
... unless I'm really missing something trivial...

u aren't using it
is it used anywhere? 
is there any perf impact?
 The attribute is used at line
https://github.com/pydata/pandas/blob/a064c7d976f5e93ea91a23f97690f1d180a03948/pandas/index.pyx#L209
The performance impact is virtually null: after the test fails, _ensure_mapping_populated ends up being called, it sees the index is already initialized and stops. So unless there are plans to make more fine grained initializations, I think one could just remove anything related to it, and make "__get__" of "is_unique" simply check self.initialized and in case call self.initialize.
 closing pls reopen if/when updated
