Vingalmo, has sent me to Riften again to bestow the vampirism among Valindor again but he already contracted this gift. when I got to his house again, he already has a coffin for him to sleep in so this means he has turned into a vampire already. Anyone know why this is happening?
This is part of a known bug that as far as my research shows, is not part of an official patch.
A workaround exists on GameFAQs:
The workaround is simple- Save your game JUST BEFORE you turn in for the reward for your current mission. If the next mission you're assigned is a Vingalmo mission, simply reload your save and it will randomly pick another mission for you. If you should randomly get offered Vingalmo missions a few times (which hasn't happened to me yet, but who knows?), keep reloading until you get another kind of mission.
If you've already gotten stuck with this bug, hopefully you have a save from before you were given the mission, in which case, just do as stated above. If not, the only options are to A) continue playing and be locked out of those missions, or B) start over (which would NOT be fun after making it that far!)
The above seems to be the generally accepted solution and definition of your experience and is referenced a number of times across the Internet.
yes its sad but you have to reload a save before you obtain this duplicated quest otherwise your stuck not being able to continue quests for the vampires of harkons castle and it would suck if your trying to get more blood potions and even the ancient vampire body parts cause you wont be able to continue on for those quests so you need to be careful