You can only find the Doppleganger after level 8. Beware, the Doppleganger is extremely powerful. He has all the skills you have, has the exact same HP and SP as you do, but he is 8 levels higher than you and all his stats are higher than yours except for HP and SP.
I reccomend you fight him at the end of game when all your characters are at level 50. I also reccomend going to an area where the Lucky Animal Sleipner is located, like Essential Red Plum's Resort. Find Sleipner and kick him. Now wait 10 minutes, don't do anything, don't even touch your controller.
When 10 minutes is up, go wander around the field. A red arrow will point towards the Dopplganger when you are near him. A message will appear on screen, then the edge of your screen will be cloudy. Run toward him and activate a fight. Right when you start the battle, Sleipner will come in reduce Doppleganger's Health by half.
Now use Beast Awakening, be sure you have Silabus and Piros for this to work effectively. Equip a broadsword and he should go down in no time. However, he is able to cast spells to heal himself, and he is constantly regaining HP ans SP throughout the fight.
By defeating him you get the Own King, a Key Item that transforms in one of the later volumes.
.hack//G.U. Vol. 1: Rebirth Hint - Defeating Dopplegangers
The best way, and easiest way to defeat a doppleganger is to create a party with Pi and Kuhn. Put their strategy on rage or free-will. Next, equip all the best weapons you currently have and choose the beast Awakening.
Go to a level 50 area with last word being "berserk", with you at level 50, build up your morale by fighting all the enemies and using Rengekis.
Equip twin-blade, find the dopplegangerand get on the bike and run over him. Activate the beast "Awakening". Finally, mess him up with continuous attacks by your whole team and you will win.
The same startegy can be used for volume 2, but use a twin-blade-Drain Heaven +5 if possible.
