Well, the only way you can be sure that an item is not a dupe is to find it yourself. But look at it this way: do you want to use dupes? If you don't, be thankful that Bliz deletes one of your duped items that you think is legit (though, given the level of duping that's gone on, I very much doubt they'll put much of a dent in the D2 dupes, or just as bad, the botted items). Personally, I spit on people (not addressing you personally ;) ) who complain about losing their "dupes-they-traded-for-using-legit-items-in-good-faith". If I had any dupes -- which I seriously doubt -- I'd gladly give Bliz one of my best legit items for every dupe they removed.
p.s. Another thing that really irritates me -- while I'm on this topic -- is people (again not addressing you personally) whose only concern about trading for dupes is that they'll disappear; no worries about cheating though. This is Stage 1 in Kohlberg's stages of moral development (if you're bad, your item will disappear -- or in the case of b.net, there is an 0.01% chance it will disappear):
http://www.nd.edu/~rbarger/kohlberg.html
Unfortunately, Kohlberg forot to include the Stage 0 that characterizes 90% of b.net.
p.s. Another thing that really irritates me -- while I'm on this topic -- is people (again not addressing you personally) whose only concern about trading for dupes is that they'll disappear; no worries about cheating though. This is Stage 1 in Kohlberg's stages of moral development (if you're bad, your item will disappear -- or in the case of b.net, there is an 0.01% chance it will disappear):
http://www.nd.edu/~rbarger/kohlberg.html
Unfortunately, Kohlberg forot to include the Stage 0 that characterizes 90% of b.net.