My Mom does the same thing (she too has ADD).... if I do it, I'm still in denial! But my Mom is notorious for it! It's kind of a funny family joke.... especially when it's the same thing several times in just a couple of days. Okay, I confess, I do the same thing on occasion.... but not that often (or maybe that's the denial again!).
I think for me, it's because I am thinking faster than I can get the thoughts out. I am already halfway through the next thought, before I even start sharing the previous thought. Then later in the conversation, I'll share the same thought again, not realizing that I had already said it outloud, because I thought past the sharing part.... does that make any sense at all??? Like I'm saying the words outloud, but even as I'm saying them, I'm already thinking about the next thing I want to say, so I think my brain doesn't even acknowledge what I just said outloud.... yeah.... something like that.
It really doesn't happen that often anymore, but I think it's because I recognized that I was doing it, so I know better now and have been retraining my brain to focus on the conversation as it's happening, and not on the next thing I want to say.
I have no idea if any of that made any sense.... I have a hollering 10 month old in my ear at the moment! That kind of drowns out everything else!!!