Normally, if you get two of the same gift, you thank each person, then stealthily return one of them and never mention the duplication. If they know about the two presents (say you opened them at the shower, for example), the official advice is to pretend to each person that hers is the one you kept.
I broke that rule, though, and got away with it! Here's why I did it (and how):
Both women approached me and urged me to return their identical coffee maker (even though I hadn't registered, they'd chosen the same model). Cousin Mariann even told me the receipt was in the box, so I knew she wouldn't be insulted. After returning the coffee maker, I wrote to Mariann, expressing my appreciation for the knife sharpener! She thought it was a hoot.
And the other lady (whom I didn't know as well) received a warm thank-you for such a well-designed coffee maker, plus the assurance that I loved her choice and had kept her gift -- which she mentioned to me in delight the next time I saw her.
Have you gotten duplicate gifts? Do you think you could be honest with the givers, or would you want to simply thank them and leave it at that?