I discovered this while trying to arrange my char's items while on the Ebon Hawk. Suddenly, I had been making duplicates of the items. I was fortunately able to retrace my steps.
Note: Duped items might be able stack if unequipped, which may destroy the duplicates.
Example:
You have bought the CNS Strength Enhancer (belt, +2 strength, +2 all saves) from the shop near the landing zone on Dantooine. You want to have it on both you and Carth. Equip the CNS Strength Enhancer on your main char and then board the Ebon Hawk. Press [U], then open the belt slot and select "None" then "OK". You press [TAB] until you reach Carth. Open his belt slot, select the CNS Strength Enhancer, then select "None" and "OK". Carth will be equipped with a fake CNS Strength Enhancer and there will be another CNS Strength Enhancer in your "backpack". Switch back to your main char and have him/her equip (select and confirm) the "real" CNS Strength Enhancer, and notice the bonus kick in. Now switch to Carth again. Open his belt slot and select another belt, then select the "fake" CNS Strength Enhancer - notice the bonusses kick in.
You're done. You'll need to repeat some of the steps to make more dupes, but it's possible.