Items duping in game should be fixed ASAP. Players in Russian region are duping items and sell it for real money in social networks.

I've heard that duping with handcuffs in 0.51 was fixed, but there are many ways to do more.

There is a way to clone the player using stairs. I don't know how it works. I saw it 2 times in 0.50 and heard, that its still works. This video contains moment of dupe that i saw with my eyes.
https://www.youtube.com/watch?v=0XvW12E-WtA at 1:50

And there is another way to dupe items if you have your own server. I dont know the true recipe for duping, I only know that they cause server restart to make dupe.