I think it would be a good idea to number the trophy pieces,so it's easier to describe to friends which part you're looking for.
Each trophy comes in 5 parts,so a good description could be,
for example: DT 3/5 (Dirt Trophy part number 3 of 5)
I tell whether a trophy piece is a duplicate based on whether or not I get the "you discovered a new item" message when I receive it. If I don't get that message, I assume it's not new... if a bug made it so that message didn't appear, I would miss out by getting rid of something rare and special, and that would make me sad... Hopefully that hasn't happened and never will, because I'm not dropping by home each time I get one to check my cabinet and compare them all...