Why dupe weapons
Borderlands 2 uses a procedural system to randomly generate loot and currently holds the Guinness world record for the most guns in a video game. With over 20 million randomly generated guns with different stats, levels and rarity it is practically impossible to get the same weapon twice.
The main weapons that are duped are the legendary weapons (orange weapons) as these tend to pack the biggest punch against the bosses and have secret advantages against other rarity weapons.
How to dupe weapons
To dupe weapons you will need to be carrying the weapons you want to dupe and 2 players.
1- Make sure that the weapons you want to dupe are equipped.
2- Start a duel with another player.
3- Put the weapons you want to dupe in to the trade box.
4- Both players need to click on duel.
5 – The player that has the weapons to dupe needs to drop the weapons on the floor.
6- Second player now needs to kill the player that dropped the weapons.
7- The second player will have now received the weapons and will be in his/hers backpack
8- First player can now pick up his/hers weapons again.
That is all that is required to dupe the weapons and takes around 1 min to complete. Below is a tutorial video where you can watch the dupe in action.