Go to the warehouse, and get the Hammer and Baseball Bat. Then, fast travel to the Church, and sell them to get approximately $400. Go back to the Warehouse, and the Hammer and Baseball Bat will have respawned. Repeat this as many times as desired. Note: You must be in the City and have discovered both the Church and the Warehouse to do this trick.

This MUST be done in solo mode: When you reach a checkpoint during any campaign quest drop the item on the ground you wish to duplicate(Can be your all your items in your inventory). Then kill yourself by using deobombs or molotovs, When the respawn timer begins to countdown pause the game and reload your checkpoint. When you respawn all the weapons you dropped will be back in your inventory and they will also still be laying on the ground where you dropped them. Side note: you also do not suffer the cash penalty from dying since you reloaded the checkpoint.

Pull out the weapon you want to duplicate and wield it. In digital fighting control scheme, hold the left trigger as if to throw the weapon, hit the Y button at the same time as you pull the right trigger to throw the weapon, if you time it right you will throw your weapon and drop a second copy at whatever the current durability of the weapon you dropped was. As a bonus keep double of your favorite weapon and use until it breaks, discard and copy the fully repaired version to ensure your never stuck with a broken weapon.