[MergeDups] Blacklist bad merge sets

[imnasnainaec]

Follow-up to #3523, which would have allowed the backend to keep returning the same bad set of potential duplicates.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 74.49%. Comparing base (25cff36) to head (8377482).
Report is 43 commits behind head on master.

Files with missing lines	Patch %	Lines
src/goals/Redux/GoalActions.ts	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3530      +/-   ##
==========================================
- Coverage   74.51%   74.49%   -0.02%     
==========================================
  Files         286      286              
  Lines       11021    11023       +2     
  Branches     1344     1345       +1     
==========================================
  Hits         8212     8212              
- Misses       2421     2423       +2     
  Partials      388      388              

Flag	Coverage Δ
backend	83.54% <ø> (ø)
frontend	66.48% <50.00%> (-0.03%)	⬇️

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[jasonleenaylor]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)