Doctor Who and Uniqueness
So, the rumors of two Tens have been flying around and that reminded me of a conversation that I had with someone a while back regarding the "uniqueness" of the Doctor. It seems that there are alt-universe!Jackie and alt-universe!Pete, and I remember wondering outloud whether or not there would be an alt-universe!Doctor. Someone else (augh, I don't remember and I can't seem to find it in any of my old emails) responded that the Doctor and the other Time Lords are unique across all the universes and dimensions. In essence, that there is only one Doctor, one Master, one Romana, etc.
Is this true? Where can I find this little nugget of information? (Although I worry that it's a piece of canon that I have forgotten or completely missed.)
I feel like such a bad fan.
ETA: Oh I found this discussion which poses the whole alt!Doctor question. It's a recent discussion, so it's not the one that I had sometime last year (or maybe even the year before, my memory is hazy (time is an illusion, lunchtime doubly so)).