How do you word a thank you note for something that you already had and returned and they know that you already had one and would return it?

Does that sentence make sense? A really sweet older couple from church got us a car seat mirror and after DH opened it, they asked if we already had one so I said yes and talked about how excited we were to be able to see the baby from the front seat. And they included a gift receipt and encouraged us to take it back, but what do I say in the note? "Thanks for your thoughtfulness and the mirror...the store credit will be really handy for baby essentials" <---does that sound rude? What would you write?