Add error checker for duplicate codesystem codes

[jamesagnew]

No description provided.

[github-actions]

Formatting check succeeded!

[tadgh]

LGTM minus one suggestion

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.45%. Comparing base (497b9f2) to head (f420b69).
Report is 99 commits behind head on master.

❗ Current head f420b69 differs from pull request most recent head 2ba4fec

Please upload reports for the commit 2ba4fec to get more accurate results.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6014      +/-   ##
============================================
+ Coverage     83.39%   83.45%   +0.06%     
- Complexity    26927    27239     +312     
============================================
  Files          1681     1697      +16     
  Lines        103965   105331    +1366     
  Branches      13189    13313     +124     
============================================
+ Hits          86702    87908    +1206     
- Misses        11613    11722     +109     
- Partials       5650     5701      +51     

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