Bug "fix" for #3932, and possibly for #3799. Booleans can get into an endless loop (at least until memory runs out); through triangulation somehow a face is repeatedly added to the list of faces to triangulate. This patch checks the face list for duplicates prior to a list add and aborts if a dup is found.
The real issue is why the triangulation is creating the face in the first place, but that will take a more thorough (and longer) examination of the code. If I can fix that issue that prior to the 2.42 release, then this code can be removed.
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