While the Alt-F4 method is still effective, and is nice for duping a lot of items at once, it seems that Duel Duping has been patched. For those that don't know, in BL2 you could duplicate up to 4 items at a time by trading with a friend, then selecting the duel option once the target items were added.

Can you still duplicate items in Borderlands 2?

You can do it in single player on PC by simply duplicating your save file (so you have twice the same character) and putting the gear you want to have twice from the duped char in your shared stash (near clap trap in sanctuary) so your main character can get it.

Did Mojang patch duping?

Yes, they just patched the Unicode written book dupe glitch in the coming version.