Dead Island annoyingly only saves at pre-determined points (though this arguably makes it more addictive). To get it to save whenever you want, go to the Controls menu and change the layout from Analog to Digital or Digital to Analog. This causes the game to autosave. You can then obviously change the controls back and carry on playing.
Note: This must be done in solo mode. When you reach a checkpoint during any campaign quest, drop the item on the ground you want to duplicate (if you want, all your items in your inventory). Then, kill yourself by using deobombs or molotovs, When the respawn timer starts counting down, pause the game and reload your checkpoint. When you respawn, all the weapons you dropped will be back in your inventory and also still be laying on the ground where you dropped them. You also do not suffer the cash penalty from dying since you reloaded the checkpoint.
Note: This trick does not work with projectile weapons. Hold LB then hold RB + Y. If done correctly, there should be two of the same weapon on the ground and it will show that you threw and dropped the same weapon. Repeat this as many times as desired. You can use this trick to make money in game or in online play, and you can supply teammates with strong weapons. You can also duplicate a weapon instead of repairing it. Note: This also works with modified and upgraded weapons.
Note: This can only be unlocked with Sam B after unlocking his Tackle ability. Play Chapter 16, "Locked Up". Once you reach the checkpoint where you have to "Find another way to the cell block control room", go around to get to the control room. When you open the door to the cell blocks, there is a stairway going down and there you will see the Ram. Let him follow you upstairs and go back in the hallway. Leave the door open. The Ram will try to ram you down, but he won't be able to get past the door. This is where you can weaken him until his life is close to dead. Then, deliver the final blows with you Tackle ability.
On Level 7, you can get a Diamond from the safe in the Mayor's office. If you go back into the sewers, but then turn round and check the safe again, the Diamond should ahve spawned back in there again. Sell the diamonds for tons of money.
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 at the same time as you pull the right trigger to throw the weapon. If timed correctly you will throw your weapon and drop a second copy at whatever the current durability of the weapon you dropped was. Keep doubles of your favorite weapon and use it until it breaks, then discard and copy the fully repaired version to ensure you're never stuck with a broken weapon.
In the hotel during the quest "BORN TO BE WILD," speak to Mike Davies. After that, move to the next room that has a workbench. Enter the next room to find one of the computers to reboot. If you look at the wall to the side of the desk with the fan on it, you will see a poster displaying "Chrome 2," one of Techland's next upcoming games.
As Logan, hold down the aim button (left trigger) and hold down the fury button while aiming at multiple zombies. Then, pull the right trigger. Instead of logan using his knives, he will be throwing your equipped weapon. Throwing the weapons will cause the game to duplicate your equipped item.
At the warehouse, get the Hammer and Baseball Bat, then fast travel to the Church and sell them to get about $400. Go back to the Warehouse and the Hammer and Baseball Bat will have respawned. Repeat this as many times as you can. Note: You must be in the City and have discovered both the Church and the Warehouse to do this trick.