We have a customer that says they were charged twice while placing an order.
Great question! Definitely not a scam, but there may be some confusion on the "charges" that are appearing. I suspect that your customer's Paypal account is attached to their MasterCard, the same Mastercard they used to finalize the payment the second time around. When a payment fails, the charge will typically appear on a mobile banking app. It is not an actual charge, but rather an "Authorization" - Authorizations appear as the credit card owner authorized the system to charge their card; however, as it failed, the charge didn't go through. Authorizations then remain on the account for 2-5 business days. The processing time heavily relies on the bank's processing times but regardless, the amount will fall off eventually. Your customer can call the number on the back of their credit card as they'll be able to confirm the first charge is an authorization and the second was a successful charge.
Hope that helps clear things up; however, if you have any questions, let me know!