I wasn't able to reproduce duplication Midge Robe glitch (green cape thing). I think it's some kind of power-on screw up.
Something I have digged up could be useful (wasn't tested myself)
Power Bonus - (reset code in manual i think..)
This trick allows your characters to permanently have super strength, speed, defense, or any other power from the girl's magic menu. Simply cast the spell of your choice on any single character, sleep at the Inn and immediately save your game. Restart, and the next time the game is loaded, your character will have that power permanently. You can do this with as many power spells as you like, but some spells may interfere with others and cause previous powers to disapear. Nevertheless, this trick is great for if your character is already at its maximum power level and you aren't satisfied with his skills.
Permanent Weapon Powers - (reset code in manual i think..)
Like the trick above, but gives the power to a weapon instead. Cast a spell on a single character equiped with the weapon of your choice, and simply go to the weapon selection menu and switch the powered-up weapon with a non- powered up weapon equipped to someone else. Try casting Lunar on the sword for constant HP regeneration.
Multiple Weapon Orbs
Have one of character pick up treasure chest that contains weapon Orb (can't be boss / regular items), switch to other character keep pressing attack button near treasure chest.
http://www.planetnintendo.com/secretofmana/secrets.html
http://mmxz.zophar.net/rpg/mana/secrets/secrets.html
http://www.flyingomelette.com/oddities/oddities25.html