I work in retail and a customer gave me a 2009 series $10 bill that has two front sides and look identical. At first I thought it was counterfeit but just found out it could actually be a misprint and it might be worth something!

What makes you think it's a misprint? The Treasury Department is unlikely to allow something like that into circulation or someone using it for a purchase.

I can't find any record of such a misprint getting into circulation.