A Rabbi involved in conversions recently told me a story about a conversion he took part in. He came to the Mikvah to take part in the Beis Din (Modern Orthodox Beis Din) and lo and behold he finds out that the Woman converting was converting with the intention to get married to her boyfriend. But not only that, but that the man she was planning on marrying was a Kohen!
This rabbi, who hadn't known beforehand what their backgrounds were (as he was trusting the Av Beis Din, a extremely prominent Modern Orthodox Chief Rabbi in Israel), asked the Av Beis Din why was this permitted.
The Av Beis Din responded along these lines: First off there is a doubt if this woman is really a Gentile, for Sancheriv mixed all the nations around, so she might be anyway Jewish. Secondly, regarding the Kohen (her boyfriend), we're not even sure that he is a Kohen, for nowadays all Kohanim are only presumptive Kohanim. So this Av Beis Din allowed them to get married based off a Sfeik Sfeika.
My question is, is this sevara or these confluence of sevaras found anywhere at all?