We would never want them to pay 2x for the same thing!
In theory, they should not be able to purchase the exact same thing twice on TpT, although they may accidentally purchase a bigger bundle that includes something. If they happened to get both things on TpT, forward the email to support@teacherspayteachers.com and ask for a refund on the customer's behalf. Also make sure to reply to the customer and let them know that we did that.
What's more likely is that someone purchased in our store AND on TpT. In this case, we can issue them the refund from our store.