Potions does damage only the first time

S basicly when a user throw a splash harming/harming II potion it does damage.
But when he throws a second potion it doesn't do damage.

When an admin throws a potions it does damage again and again until the player dies.

The PVP is enabled only in one world, we are using Dimesnion Doors.

Does anyone has an idea what might cause that the potions do damage only once?

/bump

EDIT by Moderator: merged posts, please use the edit button instead of double posting.