Because, having just reeled off a few things we really need each time we were asked we ended up with 2 gifts the same. This particular gift was the most expensive thing we asked for (though not piss takingly so! you can get them at a range of prices) and I didn't really expect anyone to get us one let alone two.
2nd person to give us one was very very miffed, but I don't see how else I could respond to the "what do you want?" question without having to think of 50 odd unique items, most of which we probably don't really need and aren't that fussed about having.
I was suitably apologetic to 2nd person and made sure they were able to take it back and get a refund.