Just today in fact! It was actually from someone who knows she is not my biological child... she said "I don't care, I still think she looks just like you!" All three kids look a lot alike, especially with similar mannerisms, expressions, and phrases they use, so I think that contributes.
I always athought it was a trip when people told me how much I look like my step-mom. One of my favorite memories was being out with my step-mom and a little girl we were babysitting, and someone telling her "oh, your girls are the spitting image of their mother!" I alwasy thought it was a funny little private joke for the two of us.
I tend to say something like "Well, she's a lot cuter than me!" or "I'm surprised you can tell us apart!" (she's 5, so it's funny). She's still young and doesn't know why she wouldn't look like me, so I don't make a big deal out of it. I'll follow her lead when she gets older, but I suspect she will see it the same way I did... since everything is funny in our family!