a) They ARE yours to re-gift--that's what re-gifting is.

b) Most likely the relatives won't remember exactly what they gave to who. I don't have a lot of nieces and nephews, and I have a good memory, but I couldn't tell you what I gave to who even a year ago, let alone three or more.

c) If they do remember and lack the sense to keep their mouths shut, you could always go with the not-quite-a-lie: "DD loved that toy so much, we wanted DS to have the fun of the same toy." That's true, as far as it goes, but leaves open the possibility that the one DD had is long gone and this one is brand new.