seeing double
I forgot the classic one, done by too many people to name check.

When mentioning I have a twin brother,

"ooo twins! are you two identical?"

I am a girl (and a big girl at that) and Weazal is a skinny skateboarding rakechild

I have actually had people do this to me when Weazal (the twin B) has been standing next to me

and this also, usually on my birthday,
"So Jaki you're (insert age here), how old is your twin brother?"
( Not just a hatstand inflamed in the membrane inflamed in the brain, Fri 7 Jan 2005, 14:44, Reply)