This is a quick guide for duping items in Resident Evil 5. Using the method you can make around 1 million gold/money within an hour. It's also extremely good for duping health sprays. I was using this method to fully upgrade all my other weapons. I got bored of doing gold-farming runs.

This method requires:
  1. Two Players (coop mode will not work)
  2. An internet connection
  3. An hosted Game
  4. Some time

Duping items in RE5
  1. Player A hosts a game
  2. Player B joins the game
  3. Player B gives player A the items he wants to dupe
  4. Player B leaves the game (don't save!)
  5. Player A starts the game and thus saves his new invetory
  6. Player B will still have his items once he left the game
  7. Repeat this process as long as you want

Items to preferably dupe
My recommendation is to go for stackable items. The amount of items duped increases exponentially and thus you should go for stackable, nicely selling items.
  1. Golden eggs (2000 each)
  2. Red+Green combined healing sprays (500 each)
  3. Healing Sprays (500 each)
  4. Grenades (500 each)

Also check out this YouTube video showing the process:
http://www.youtube.com/watch?v=YQWilHl2c24