Cross-shipping: I sent the wrong dead board, got working upgrade back

So I had this supermicro board that died right took it out of my old Contabo box that I got from a friend lol anyway I rma'd it to supermicro but I guess I grabbed the wrong one from my pile of dead boards I sent them a x9 something instead of the x10 I actually had the ticket for two weeks later I get a box back and its a x11 working board with ipmi and everything way newer than what I should have gotten I didn't say anything at first because I was just happy to have something working but now im feeling weird about it its been three months should I tell them or just enjoy my accidental upgrade

The silence has already cost you three months of compound guilt. At this point the ethical calculus gets interesting. You've formed what's essentially an unilateral contract with your own anxiety (like this), and it's billing you monthly. Semicolons aside, the decent move is a quick email to their support. Worst case they shrug, best case you sleep better. I've seen vendors write off far stranger inventory discrepancies. They might just tell you to keep it and feel good about doing the right thing. Dry humor doesn't apply here—this is genuinely just a board in a box, not a kidney.