Thank you so much for that information!
Well, in Croatia both
Vanja and
Sasha are used as unisex names, so I's not hard to see where Sanja would also go under this category (although Sanja is exclusively female here).
It's also possible that it was created as a mix of
Sonja and
Tanja here, since both names were very popular at the time Sanja came into prominence.
Add the fact that Sanja is a word name here, I doubt there even is a clear answer.
BTW, there's a male form in use here - Sanjin.
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