Issue/vrz 007 unique validation

[stevenbal]

fix for: VNG-Realisatie/gemma-zaken#1693

[codecov-io]

Codecov Report

Merging #13 (ea97d3f) into master (9566830) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   88.59%   88.77%   +0.17%     
==========================================
  Files          60       60              
  Lines        1859     1888      +29     
  Branches       47       48       +1     
==========================================
+ Hits         1647     1676      +29     
  Misses        198      198              
  Partials       14       14              

Impacted Files	Coverage Δ
src/verzoeken/api/serializers.py	96.38% <100.00%> (+0.04%)	⬆️
src/verzoeken/api/tests/test_verzoekproduct.py	100.00% <100.00%> (ø)
src/verzoeken/api/validators.py	90.00% <100.00%> (+1.53%)	⬆️

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