The item duplication trick works as follows:

Suppose you want to copy a Knight Sword like Excalibur.

Go to a shop that sells swords, equip a character with Two Swords ability.

Equip the shield in the right hand and the sword in the left (in other words, the reverse of the usual positions).

Go to the shop and ask for 'best fit' A second copy of the sword should appear in the right hand.

Agree to pay for it and quit.

Your character now has two copies of the same sword!

Note: this bug does not work with all weapon types.