To allow Adol to equip two of the same type of accessory, first have five additional characters in your roster. Then at Comodo craft SIX copies of the accessory. Equip one accessory on each of the characters in your party in the FIRST accessory slot EXCEPT Adol, who should equip it in his SECOND accessory slot instead. Craft one more of the same accessory, then when complete choose to equip it onto one of your characters. If you prepared your party correctly, Adol is the only character who will be able to equip the new accessory (the rest of your party members should not be selectable because they have equipped the accessory in their first slot already).
This glitch is especially useful if you wish to basically double the effect of a particularly useful accessory on Adol.
Note: If you ever unequip the accessories from Adol, you will need to craft another copy of the accessory and make the game equip it onto him again (remember to have the same accessory equipped onto your other party members first as described above). That's because this glitch takes advantage of a quirk in the post-crafting confirmation screen.