I'm doing the quest New Allegiances, and I have already done this and yet I have to turn the person yet again, but I can't.

The person is random, just reload a saved file.

I did and It's the same person! Anyone else with this problem?

Yes I have. Below is copied from another thread that I was talking about this in:

I'm stuck at 6 days on my blood chalice. I tried to pick up more quests from the people around the castle and I got a quest to turn a guy into a vampire that I already turned into one. When I go try to turn him, my vampire seduction doesn't work and he says something like "we're brothers now, etc." And when I go back to the castle to try to get the further blood chalice quests everyone just says "talk to so and so (the guy who gave me the quest to infect the dude)." It's bull.

When I hit him with vampire seduction he says something to the effect of "Oh i didn't feel anything. Did something happen?" or something like that but he still glows like the spell worked. But then when I go to like talk to him/feed on him it the box that asks you if you want to feed or talk to him doesn't even pop up (you know how there's that like pre dialog box thing-just like feeding on a sleeping person as a vanilla vampire). He just says some generic response. That's a good idea to try to kill him. I've give that a try. Thanks!