Once they do an inventory, determine they are short they will look for things like duplicate shipments. Legally they can charge your CC as you received their property and didn't bring it to their attention.
If you don't want the material they will instruct you to leave it on your front porch, or with a neighbor for pick up the next day. UPS and Fedex have services exactly for this.
If you want a back up I'd explain the situation and they will extend you a huge discount that will make you happy.
Remember that their job isn't to hastle you with unexpected shipments - but to make you a satisfied customer.
Legally they CANNOT charge you for something that they sent you that you did not order. That's mail fraud. Got a free Fender P from zZounds that way.