1. Take one weapon (for example, Mosin rifle).
2. Take weapon you want to copy in your hands (for example, geared M4).
3. Leave the server.
4. Rejoin your server as soon as possible.
5. You will find а copy (dupe) of your in-hands weapon on the ground nearby.
6. PROFIT! :)
NB: Look at the screenshot! Don't try to pick weapon from the ground after rejoining. Instead scroll inventory down and drag'n'drop weapon you'll find there to your hands (you can see it in the right part of screenshot). Only after than you can manipulate weapon on the ground as you want (detach parts, take in hands etc).