Ensure name uniqueness in filter set

[tonioo]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 95.06173% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 87.16%. Comparing base (5c6e483) to head (77c1ba6).

Files	Patch %	Lines
sievelib/factory.py	92.72%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
+ Coverage   85.79%   87.16%   +1.36%     
==========================================
  Files           9        9              
  Lines        1908     1955      +47     
==========================================
+ Hits         1637     1704      +67     
+ Misses        271      251      -20     

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