Hello! I'm the Doctor!
No, I'm not any specific doctor. It's just the Doctor.
I travel through time and space in a wonderful little time machine I call the T.A.R.D.I.S- that's Time and Relative Dimension in Space. Well, actually, I wouldn't say that it's little. It's quite a bit bigger on the inside.
Anyways, if you see anybody else that claims to be me, they probably are me. I've got a lot of alternate selves, past regenerations, and even future regenerations I'm not even aware of yet! Therefore, none of them are simply posing for me. They're all me, and I'm all of them. Don't try and question the fact that different versions of me are scattered throughout time. It'll most likely confuse you.
Oh, by the way, I happen to be looking for a companion. Someone who will travel through time and space with me. If you're interested and you see a blue police box sitting anywhere, try knocking on the door. As long as you promise not to cause trouble, I'll let you in, and off we'll go, exploring everywhere and every time period that has ever existed! Allons-y!
People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint - it's more like a big ball of wibbly wobbly... timey wimey... stuff.