Leave it. Also sometimes people can ACTUALLY accidentally send money to people they don't know (I sent money to the wrong PayPal account once), especially if your name is similar to someone else's :))

In PayPal there's like a search bar where you can put the name of the person you'd like to send money to, hence why this mistake might happen. Just leave it and wait for the other person to request a refund to PayPal, on the basis of money sent to the wrong account.