Understand that the 'backup shopper' listing is in reality just a way for the software to continue taking applications after the shop has been assigned. It's not as if you are assigned the backup shopper designation and guaranteed the assignment if the original shopper cancels.
If the shop is put back on the boards, then all of the original applications remain, and the ones entered as a backup are included as well. Put in the application as you would for any assignment that was available....