Fix possible duplicated matches on some strings

[vthib]

If a single variable produces multiple literals from which the same atoms are extracted, multiple matches can be wrongly generated for this variable. This is fixed by a unicity check before creating the Aho-Corasick.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.34%. Comparing base (cf2b7a4) to head (b62776a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #244   +/-   ##
=======================================
  Coverage   98.34%   98.34%           
=======================================
  Files          95       95           
  Lines       26758    26762    +4     
=======================================
+ Hits        26315    26319    +4     
  Misses        443      443           

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