If a customer makes a purchase at another Square seller and the receipt comes to you, there's a good chance they mistakenly input your email address during an early purchase with a Square seller. Your best bet is to unlink your email address from their card. You can do that by clicking Not Your Receipt? at the bottom of the receipt.
This article has more info: https://squareup.com/help/us/en/article/4989-incorrect-receipts
