Any kind of multiple birth (twins, triplets, etc.) involves greater risk in fetal development. They say that miscarriages are often a mis-formed fetus that couldn't survive as it was. It's very sad.
My cousin has 3-year-old identical twin boys. Twins pop up in my family every three generations or so.
But to answer your question, Jeff, yes, that's how it feels on the other end. I never went through that with you and Greg for the same reasons Fuzzy said--I got to know you before I knew Greg, but I knew of him and once I did meet him a few weeks after I met you, you had different glasses and different voices, and Greg was a bit heavier than you, too. So there were enough differences that unless I just really wasn't paying attention, I could tell you apart.
That same year, though, there were twin girls in my ward who I could NEVER tell apart, and I'd always run into them on campus either together or alone, and just couldn't ever call them by name for fear of getting it wrong.