Prevent list of trained model names from having repeats

[landoskape]

In add_model() of the io module, the list of user model_names is appended to with the new name, even if it already existed. This is confusing, because in the select custom model section of the GUI, it will repeat the name, even though only one model under that name already exists. This corrects that by checking if the fname of the new model exists in models.get_user_models() before adding it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4f56619) 63.41% compared to head (ecad632) 63.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #839      +/-   ##
==========================================
+ Coverage   63.41%   63.50%   +0.09%     
==========================================
  Files          14       14              
  Lines        2971     2976       +5     
==========================================
+ Hits         1884     1890       +6     
+ Misses       1087     1086       -1     

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[carsen-stringer]

thanks!