Mid-story deja vu, what do you do?

Stop, and apologize

I'd mention that I think I've told it before, because I don't want to seem oblivious. Then I'd keep telling it because if it is a good story, it is worth telling twice.

I'd stop, although I'd say something about having told it before, and I might say something about why I was telling it in the first place--maybe the ending was relevant in some way.

Change it on the fly and see if they notice by saying that's not how it goes.
I love messing with peoples heads

Mention that I think I've told the story before and stop if my listeners confirm they've heard it.

The closest option was stop, but I would say, "Have I already told you this?"

When I try to repeat a story to my kid, she stops me.

Mention to them that you think you've told this story before, apologize but keep telling the story until the end

I usually trail off a bit and then tell the rest of the story very quickly in order to come to a conclusion or make the point I was trying to make in the first place. If they make it clear to me that they recall the story (by saying so or through body language/expression), I'll just move on. I have a couple of friends who will retell it with every detail, even if someone tells them that they remember being told before - it's a bit annoying!

Other: Ask if they've heard this before. If they have, stop telling the story.

That happens to me a little too often. I just can't quit once I've launched into an anecdote. More often than not, I'll admit that they've likely heard it before, but continue with an abbreviated version.

I'd apologize and stop. I find it annoying when someone tells
you a story more than once, but usually I will just humour them.

I would stop and apologize/ Some people get angry hearing the same thing.

Keep going..

It's funny how people don't like hearing the same story retold, but they will watch movies more than once and reruns of shows.

Good point. I can act

No big deal for me, I am often to be found telling the same story more than once, I never get any complaints...mine are good stories that bear repeated re-tellings. And I can't see myself telling them the same way twice.