I was very excited when one of my families finally had a set of twins. They were two boys with light brown hair, each wearing a red track suit. But, imagine my surprise when the mother has another set of twins right after them...with the same head/outfit as the previous set. It's like I've got quadruplets running around. I can only tell them all apart by name.

Has this happened to anybody else?

In one family, the mother had blue hair, father was mouse blondie, they had six single kids, and four of them were identical, purple haired little girlies. For the next gen, I picked one of those girls.

I sometimes change their clothes so I can tell my multiples apart.

This is off-topic, but, my aunt used to do the same thing for her real twins. And sometimes they exchanged their clothes just to fool their mother

How you do it? My couple just give birth for 1 baby gal and adopt for another gal. Right now i want the 3rd. Put them together they start to argue. T_T What can i do?

You can ignore the arguing and keep trying. The only time you absolutely must wait is if they say it's too soon (or if they already have 6 children).