I'm with lonemagpie: it's an awful big universe, and there's only so many faces out there. Even with the limited number of people ordinary folks like us meet in our ordinary lives, once you get past thirty-ish you've met enough people for it to be increasingly common for people to remind you of someone you used to know. For someone like the Doctor who's meeting new people every landing, it must be normal life.
If anything, what bothers me is when it happens in the likes of Lewis and Midsomer Murders; you're a professional detective, Lewis, who could instantly spot that someone was living under an assumed identity as you recognised their face last week, yet now you don't notice that the woman in your current case was the accomplice in the case that killed Morse? (Sorry, spoiler!) ;-)