I once wrote a thank-you note ina similar situation. It went something like this:
"Thank you so much for the electric knife sharpener--I've been wanting one for a while, since I already have some great knives. 'What knife sharpener?' you say? 'I gave you a coffee maker!' That is correct--but as you know, I received two identical coffee makers, and as you pointed out, nobody needs two. Since you suggested it, I exchanged your coffee maker for the knife sharpener, and now every time I sharpen my knives, I think of you warmly."
She told me later it was the best thank-you note she'd ever gotten.
I like kckgirl's suggestion.