Tenant claims he's paying the rent to someone else

[Stewart VanValkenburg]

So I have a duplex in Binghamton NY. Switched out property managers last year because the one I had had sticky fingers. Found there was someone living in a unit who said it wasn't the old PM who put him there or the new one, but someone else. Claimed to be paying rent to this other person. Fast forward a year and he's still there. My new PM wanted to wait till his lease was up (Aug 31st) instead of going to court because court is a multimonth process there and it was trickier than usual with his claim. He's still there and trying to collect rent from the other tenant in the duplex. Have you ever had this happen? Anything in particular I can try besides sic a lawyer on him?