Surely the old owner of the said account must have closed the account, therefore, they are the one at fault as they must have known they'd closed the account.
I've got an account that I haven't used for 10 years or more (it has about 42p in it) and I don't expect the bank to just recycle the account number until I notify them or they notify me that they're closing the account and sending me the money in some form or another.
However, I'm not saying that you shouldn't pay the money back - it wasn't yours but you made a mistake.
I would be careful though because if the person with the old 10 year acct thinks they can still put money in the account, maybe they think thay can talke money out of it!!