add TestConcurrentBufferDuplicateKeys

[leeym]

Adding a new test TestConcurrentBufferDuplicateKeys and fix ConcurrenfBufferMemcached to make sure it follows the other implementations to actually act as a set and handle duplicates correctly.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.46%. Comparing base (9fa00a2) to head (4445182).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #35 +/- ## ========================================== - Coverage 83.51% 83.46% -0.05% ========================================== Files 10 10 Lines 1468 1464 -4 ========================================== - Hits 1226 1222 -4 Misses 169 169 Partials 73 73 ```

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[mennanov]

Feel free to merge when ready

[leeym]

This branch has no conflicts with the base branch

Only those with write access to this repository can merge pull requests.

It looks like I don't have write access to merge it.

[mennanov]

Thanks!