Read at your own risk.
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Well Just for fun I decided to see if you could duplicate items the same way in BD as you could in DD and you can.
Basically this is a simple inventory file swap.
How to do it: Throw everthing on the ground that you want a duplicate of, save the game as "I am a cheating bastid" or something of the like. Exit the game and go to your savegame folder for BD. Go to your last saved game where your inventory is all still intact and copy the 6 inv files to your "I am a cheating bastid" save game folder. Now when you load your bastid game everything you threw on the ground will be in your inventory as well as on the ground.
Things I have noted so far:
The gem bag if you have a bunch of duplicates they all stack and you can have major resistance.
I was hoping that if I had 3 or 4 duplicate summoning dolls that you could call 3 or 4 of them up at once. Didn't happen. If you were inclined though you could have a duplicate set in your other PC's inventory and either of them can call em up.
I also tried copying Dollinfo from the savegame directory to an old old saved game to see if it would alter the summoning dolls in the old game but nothing happened there either.
I tried copying a data file over to a dif save game as well and got a fantastic blue screen when I started up BD, I just deleted the save and all was good again.
If anyone else decides to do some experimenting I would like to see your results. But don't have your characters come running and crying to me if you accidently delete there legs and eyes and ruin your save games. Always practice on duplicate saves.