This is one of a couple Doppleganger fights that are mandatory in the game. You choose two other members in the grou7p to fight with and then dopplegangers for the partners come up and you fight them. After you defeat them you cant get them back because they tell you to go on and they will take care of the dopplegangers themselves.

.hack .hack//G.U. cool G.U. vol.3 vol3