Basically you get someone to make a game, then once you drop whichever weapons you want to duplicate you ask the host to boot you, which will cause the game to not save giving you your gun back and letting the other person have that gun. If you want to duplicate a weapon to sell it I would guess you would have to get the duplicated weapon back from the other person.
Random.
You cant make weapons in borderlands, you find them, (i finished game)
rider and something else
Crawmerax cannot drop gearbox weapons
You keep all your weapons and skill levels when you start the second play through.
you have to find them orget them from qeusts (i get alot from red weapon cases)
go to the gun shop at the bottom there should be gun mods. i think
Usually in weapons crates. Purple and orange weapons are the rarest. You find rarer items in the second playthrough.
They r in all of the gun cases, It will just b a luck of the draw to get them....
There are vending machines at certain places where you can sell any unwanted loot and buy new weapons.
Weapons are randomly dropped, there is no way to say for sure where a specific combination can be found.
You can find corrosive weapons all over the place. Check the gun vending machines located all over Pandora, and when you look for corrosive weapons, look at the text description of the gun, and it will tell you if it is corrosive or not.