Whoa! That’s the first time I heard of THREE payments being sent. I’m assuming that since you said “we”, you are not talking about one for each of you, and one extra?! This is pretty strange, and very gray territory. I would feel free to use/spend the first deposit you rceived, but move the other one to a money market account separately, and also deposit the second paper check in that MM account. Then just wait. I’m sure they will come after it at some point — maybe even when you file next year’s tax return. So don’t spend it! But I have no idea how you would return the money and be sure they properly credit it to your account!
This wouldn’t fall under the “no clawback” rule, I’m sure — since it was obviously a clerical mistake, not a calculation mistake.