When a girl reminds you of an old love?

I met this girl recently, who has surprised me with how much I have chemistry with her. The thing is, there's been a few times where she's said something funny and I start to say "that's funny" and almost call her the name of this girl I had a great friendship/relationship with, because she would banter with me in the same way and make me laugh out of nowhere (things only ended because we couldn't make a long-distance work). I don't really know what it means, but it's made me think about the first girl a lot. What should I do to stop this and just focus on the girl I'm talking to?