Avoid duplicates in select menus (with yadcf)

[pankus]

... this small piece of code works quite well for my needs, but I can't figure it out whitin different situation

[codecov[bot]]

Codecov Report

Merging #102 into master will decrease coverage by 0.45%. The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
- Coverage   93.68%   93.22%   -0.46%     
==========================================
  Files           1        1              
  Lines         190      192       +2     
==========================================
+ Hits          178      179       +1     
- Misses         12       13       +1

Impacted Files	Coverage Δ
datatables/__init__.py	93.22% <50%> (-0.46%)	:arrow_down:

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[tdamsma]

This seems like a good change, but there is no test coverage for this. @pankus , do you feel comfortable adding a test that reproduces the desired behaviour? I don't have time right now to look into this, but will do eventually

[pankus]

I can try, even if I don't know where to start !!