Fix #2182: avoid double accumulation in *MixedDuplicated

[danielwe]

I added nonzero dvals to all the tests that hit any of the branches within the modified code block. Everything passes with this change. However, I don't understand the wider context of this code very well, so please review carefully and consider whether this is the appropriate fix.

It would probably be a good idea to systematically add nonzero dvals to most/all reverse mode tests, but I'll leave it here for now.

Fixes #2182

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.36%. Comparing base (037dfed) to head (9360f38).
Report is 333 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2262      +/-   ##
==========================================
+ Coverage   67.50%   75.36%   +7.85%     
==========================================
  Files          31       56      +25     
  Lines       12668    16734    +4066     
==========================================
+ Hits         8552    12612    +4060     
- Misses       4116     4122       +6     

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