Duping!

is a bad idea, right?

There have been previous glitches before where it has been possible (or it was either fake when I watched them) or they are just visual tricks people do.

Steam Support.

Steam accounts have been banned in the past as a result of taking advantage of a TF2 bug where duplicating items is made possible. Most level 1 and 7 Max's Severed Heads are duplicated, and are therefore worth far less than that of any other level.

It has been done, but shouldn't be done. If you find a way to duplicate items, report the bug to Valve and you just might get rewarded for doing so.