They fixed these bugs long long ago. Circa 1.10 patch.
You can still dupe something now, but the server will erase the duped item when you exit and join another game.
As far as I know the "temp perm" method still works as in you opening trade then leaving the game promptly after trade is closed.
It's possible, but from what I understand, these trade tricks stopped working in the later patches. I also heard that some sort of internal ID is attributed to items which is specifically used by the server to detect duplicates. I also remember Blizzard doing sweeps on all characters on a realm and purging duplicate items every so often in the past...I doubt that is necessary now.
What scares me is how they originally used to do this. Some clever people figured out how to bridge an open and realm game a long time ago. This made it possible for an open character to give, say, large numbers of sojs to the realm character. Of course, this doesn't work anymore. Some godly items might have also been manufactured and transferred to realm characters.
