BUG, TST: Check uint64 behaviour in algorithms.py
First of a series of PR's to patch and test `uint64` behaviour in `core/algorithms.py`.  In this PR, the following functions are checked:

1. `duplicated()` : robust but now has test to confirm
2. `mode()` : robust but now has test to confirm
3. `unique()` : non-robust but patched and tested
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14934?src=pr) is 84.65% (diff: 100%)
> Merging [#14934](https://codecov.io/gh/pandas-dev/pandas/pull/14934?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14934   diff @@
==========================================
  Files           144        144          
  Lines         51030      51043    +13   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43199      43213    +14   
+ Misses         7831       7830     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [74de478...6d31057](https://codecov.io/gh/pandas-dev/pandas/compare/74de478392e09cf938d244f5990da4e001afc84c...6d31057d0b1b9bb6f2840771a9aff3860f2b9b00?src=pr) @jreback : Tests are green.  Ready to merge if there is nothing else. @jreback : Addressed all comments, and everything is green and passing.  Ready to merge if there are no other concerns. few comments thanks!