I understand how nervous some people feel about alts. I remember a time when alts were viewed as a bad thing, because people used them for not-so-nice reasons.
So I decided to establish a personal "rule" that if someone who met me in one alt approached me in another alt, I would admit that I've met them before. But they would have to say hi (or something) to me first. I would not approach them.
The first two instances were fairly insignificant. I don't remember the first one, but the second one met one of my anonymous alts first then another anonymous alt. No damage done.
The third instance was with Impy. He met one anonymous alt, then he added Opal in a social media site (not SL). I considered that as an approach to another alt anyway. Needless to say, that was a significant coincidence.
Today, someone saw one of my anonymous alts and complimented me. I said thank you, of course. But, as I was reading his profile, I realized that I had met him in real life at SLCC 2011! And I even took his picture! Anyway, I had to tell him. My rule. :) But he first promised to keep my secrets. :)