Why do I have to go to Riften to turn Valindor into a vampire when I already did that previously?

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

If Vingalmo gives you a quest to turn someone who has already been turned, and reloading is not an option, KILL the vampire npc Vingalmo sent you to double-turn. You fail the quest, but you can pick up new vampire quests after that. Return to the vampire castle. Save your game. Get a different randomly generated quest by asking one of the castle vampires how you can help. If they send you to Vingalmo again, load your save and try again until you get a good, non-bugged quest. Vinglamo is baaad! I don't trust him either now!