Either Faron or Xel'Naga,( or possibly someone else) should make a detailed post about dupes so it could be Stickied. Cause I'm still kind of confused about it.
I could probably put something together at some point. The problem is that Valve's trade API is difficult to work with, and since trades between private backpacks are impossible to track (which is where a lot of dupes happen), it makes it difficult to tell with 100% certainty what is or is not an actual duped item.
For instance, the Max Head I was looking at with Zombie had a very recent creation date (which is obviously a dupe), however TF2Items doesn't seem to link it to any items with the same original ID. This leads me to believe that maybe a new dupe bug exists, which creates a new original ID (which will make tracking many dupes near impossible), or perhaps Valve has changed their process when they restore items, and they make restored items with a new original ID (which is also problematic, as this means that it's even harder to track duped hats).
The bottom line is that I could write out an incredibly detailed guide for spotting duped items, and it could go obsolete within days, since Valve is constantly changing their own processes for dupes, and since the trade API is so wishy-washy.