We don't have twins here, but I thought this thread was great.
I did nanny for 3-4 month old twins a few years back, when my eldest was 1.5 or so. I used to take them out for walks, or for errands, or to meet their parents at the clinic for WBV's and such, and I'd set up my ds's and one of their boys in the dbl stroller, and sling the other baby. As luck would have it, the twin that liked to be sling was their puerto rican twin - the other twin was white as can be, just like my son. I used to get the weirdest comments - they'd assume that my 30 lb ds and the 10 lb lighter twin were twins, and the other baby was the adopted. I was asked if they were triplets, someone asked if I was a foster parent, someone asked if the darker twin was the nanny's (LOL), and one person fought me, said there was no way the twins were actually twins, since they were of a different race. I tried explaining that Dad was Puerto Rican and Mom was white, and they were just not having it. Once, I said that I was just the nanny, and someone was like "oh, you're a nanny for 2 families! How nice that the have babies so close in age! Looks like those parents (gesturing to my ds and the light twin) need some birth control, if you know what I mean."
Mind boggling the things people come up with.