I feel guilty about this issue. Last time I went out with my friend and her dd, for some reason everybody thought they were ALL mine, and I had triplets (I shudder to think--two is more than enough!). I don't know what she thought about this, because she kept her mouth shut. I thought it was kind of amusing. But I think maybe this is a sensitive issue in general because it strikes that "she's more of a mom than me" nerve. We're all moms doing the best we can, but I think there is a certain perception that twin moms are Super Mom. To be honest, when people call me that I don't really protest. It feels good to hear that when I'm working so damned hard, and especially when I'm feeling sorry for myself. And sometimes the jealousy works the other way: I wish I could go out as easy as singleton moms, and not be so fearful about a scene in public, only have one diaper to change, etc. etc. I really have to watch myself that I don't whine too much to my friends about various difficulties with dealing with the twins, because they can't relate and I'm sure the martyr trip gets annoying. This is where twin mom groups are helpful, because you don't feel like such a sideshow and they can relate to your experience!
On a personal level I can relate to what 1Plus2 said, because my sister had to take Clomid to get pregnant after many miscarriages, and I think the news that I not only didn't have trouble but was going to have two must have been a little...difficult.
I get the "oh I always wanted to have twins" comment all the time." (Suuuuuuuure!). My response now is to ask them to babysit