Dear Amber,
1) 2 mail to addresses on any package or letter will be returned for sure.
2) you're right, it could be one is a ship to address and the other one is a residential address.
3) Do convo the customer and show him/her the 2 addresses you have and ask which one they'd like you to ship the item to, as Ariella said.
Let us know when you have another sale so we can celebrate with you.