I´ve completed the toad-quest for the warriors ugild in Valeia with one of my characters... I then joined the guild with two others and now they´ve received the same quest. Is it possible to complete this one more thaan once?!?

Yes, it just takes another visit.

Note that talking to Ekbu is not enough: you at least need to talk to shinwiki. For safety I also touched the toadem again.

I think just entering Shimwiki's hut is enough; the first time I played, I never found the lost toad to give a potion, so I never learned Ukabu and had to fight my way through the place and kill Shimwiki, but all characters w/quest got credit...