Do the following at the main menu or at a Training Hall:
1. Save the game and remove one member of your party, so you have five remaining. Create a new character, for example "Bob", and add him in. Remove all of his equipment.
2. Transfer all the equipment you want duplicated to Bob. After that, remove him and quit the game completely -- do not save.
3. Reload that game to return to your normal six party members, with the items you intended to copy. Remove that character again, and add in Bob again, and he will also have those items. Trade those items back to the party, and your party will have their items, and the duplicates that Bob had.
4. Remove Bob and add in your other members, then save. You will be back into the game, with two sets of items.
5. The character duplication trick is far easier, but only if he or she has not been in combat and can be modified. Rename your character using the "Modify Character" command, then remove him or her. Quit without saving, and reload. Your renamed party member will be in the pool of characters, and your original character will still be in the party. This is also a great way to gather up useful items from new characters in Dark Queen of Krynn, where some of them receive potent magical items not found in Death Knights of Krynn.