fix: Catch invalid model specification with duplicate channels, samples and modifiers.

[lorenzennio]

Description

Fixes #2059 but extends to channels and samples. We want to catch the case where we have multiple channels with the same name in the model specification, where the first of these would simply be ignored as of now. Similarly, this applies to samples with the same name in a given channel or modifiers with the same name and type in a given sample.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a1a31f1) 98.28% compared to head (edea8ef) 98.28%.
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2396   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
=======================================
  Files          69       69           
  Lines        4539     4556   +17     
  Branches      803      814   +11     
=======================================
+ Hits         4461     4478   +17     
  Misses         45       45           
  Partials       33       33           

Flag	Coverage Δ
contrib	97.87% <100.00%> (+<0.01%)	⬆️
doctest	60.66% <47.05%> (-0.06%)	⬇️
unittests-3.10	96.31% <100.00%> (+0.01%)	⬆️
unittests-3.11	96.31% <100.00%> (+0.01%)	⬆️
unittests-3.8	96.33% <100.00%> (+0.01%)	⬆️
unittests-3.9	96.35% <100.00%> (+0.01%)	⬆️

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