Hmmm, strange. Your customers should only be receiving one receipt for purchases! 

If a customer has visited a Square seller before, they may start receiving Automatic Receipts. You would know if this is the case when the app skips the receipt screen, and you don't need to enter a preference for receipt delivery.

It may be an issue if your customers are receiving two for some reason. If you'd like for us to take a look a particular transaction, go ahead and contact our Support team.