I'd add that this may have impact on divorce proceedings - although I say that without firm legal knowledge.There is in fact an immigration matter here. If OP can concretely prove that the spouse was married to someone else at the time they got married, this is major misrepresentation and means that both PR and citizenship were obtained fraudulently. Whether CBSA and IRCC pursue this is an entirely different matter. However you are very much mistaken if you really believe there is no immigration issue here.
Revocation of citizenship is possible in these circumstances.