1) Go to a bonfire.
2) Use the bonfire.
3) Empty your head slot.
4) Equip the Grass Crest Shield or Cloranthy Ring in the head slot.
5) Equip the Dragon Head Stone with the soul or item you wish to duplicate directly after it.
6) Face forward, and use the Dragon Head Stone again while holding X.
7) Straight after, turn around while holding X, and press Down on the D-pad to swap item.
8) Release X for a fraction of time, then hold it again.
9 If you did it correctly the item will of duplicated.
10) Repeat as many times as desired.

This may sound quite difficult but it will become easy with practice.