Fix types/findDup()

[alessio]

The function does not work as expected - denoms need
to be compared, not coins.

Add unit test.

Thanks: @zhuliting for pointing this out

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added a relevant changelog entry: clog add [section] [stanza] [message]
• rereviewed Files changed in the github PR explorer

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[alessio]

Issue was pointed out in #4302

[codecov]

Codecov Report

Merging #4303 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4303      +/-   ##
==========================================
+ Coverage   59.12%   59.12%   +<.01%     
==========================================
  Files         217      217              
  Lines       14605    14606       +1     
==========================================
+ Hits         8635     8636       +1     
  Misses       5332     5332              
  Partials      638      638

[alexanderbez]

We should add a pending log entry stated NewCoins is fixed by way of fixing findDup.

[alessio]

@alexanderbez done, thanks