Add ShouldBeUnique

[rmcdaniel]

Fixes #277

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (aa9424f) to head (d0fc48b).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##              master      #278   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       310       312    +2     
===========================================
  Files             46        46           
  Lines           1297      1301    +4     
===========================================
+ Hits            1297      1301    +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.