Okay, you have a rare item, and you wish to duplicate it. Follow these steps for an easy way to dupe :)

First, look straight in front of you, but make sure no one is in front of you or you will hit them and get a bounty. Swing your weapon, or punch the air. But before your attack is done, you need to quickly open your inventory. Try to equip another weapon, it will say "You cannot change weapons while attacking".

Then, go to a certain type of scroll, but make sure you have 2 or more. Press A, and then drop the item you wish to duplicate. Then, exit out of your inventory. Your item should be coming out of your weapon as you swing it.

And there you have it!