retrace: move the duplicate checks and bug updates into helper functions

[bdrung]

Pulling out the low hanging last commit of #297

[codecov]

Codecov Report

Attention: Patch coverage is 7.14286% with 26 lines in your changes missing coverage. Please review.

Project coverage is 84.22%. Comparing base (ccb7242) to head (5f5bbfb).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
bin/apport-retrace	7.14%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #542      +/-   ##
==========================================
- Coverage   84.23%   84.22%   -0.01%     
==========================================
  Files         104      104              
  Lines       20789    20792       +3     
  Branches     3254     3254              
==========================================
+ Hits        17511    17513       +2     
- Misses       2826     2827       +1     
  Partials      452      452              

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[bdrung]

tests/system/test_apport_retrace.py tests apport-retrace by calling it and coverage does not see that. So I am ignoring the code coverage complain.