most of my charactors had joined their guilds(1s available in valeia). when i was doing guild quests for my worrior classes(i have 2 worriors in the group), i was asked to go to toad village to investage stuff, so i did and quest completed for 1 of my worriors, i asked for a quest for my second worrior, and the same quest was required of me, i wasn't too sure 'bout this, but i travled to toad village again anyway, and luckily the quest was also accomplished.

That raises the question for a quest for my 2 wizard class charactors, i was asked to kill some guy in the serpent temple, am i meant to kill the guy twice??

Nope. Any quest that involves killing something or retrieving something can only be done once. Any quest that involves finding out information or delivering something can be done multiple times.

So if you get to a quest like killing Tevik Teporn, make sure everyone who can has the quest before you head over there. Otherwise, they all lose the chance.

I think Bungelau answered that right. If you want a charactor to complete one of thaose once off quests makes sure that all charactors, that can, have that quest as well.