ERR: qcut uniquess checking
- [x] closes #7751
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Add option to drop non-unique bins.

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14455?src=pr) is 85.25% (diff: 75.00%)
> Merging [#14455](https://codecov.io/gh/pandas-dev/pandas/pull/14455?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14455   diff @@
==========================================
  Files           140        140          
  Lines         50631      50633     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43166      43168     +2   
  Misses         7465       7465          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [c31ea34...4450884](https://codecov.io/gh/pandas-dev/pandas/compare/c31ea341747ac40d83e64d00bac179548e4b716e...445088448f1f33ec068839575a17c9bbc1ebdd8e?src=pr) Thx for the PR. Can u add tests and whatsnew?
 looks pretty good. some more error checking formatting is needed. pls add a whatsnew note in 0.19.1, make an Enhancements changes section. Be sure to say that the default is the existing behavior.
 pls add some tests. use the example from the orginal issue exercising both options to `duplicates` and pass some invalid values as well (and assert those raise)
 Thanks for the feedback -- this is my first PR on an open source project -- will make the changes and resubmit tomorrow. Had some trouble building my branch on Windows.
 contributing docs are [here](http://pandas.pydata.org/pandas-docs/stable/contributing.html); there is a section on creating a windows env.
 can you rebase @ashishsingal1 something went wrong with your rebase. Can you do:

```
git fetch upstream
git rebase upstream/master
git push -f
```

That should normally solve it Trouble rebasing, going to start over with a new PR.