It's happened, yes.
I had two female mice; a brown one named EIlonwyvern and a black one named Bula
![Stick Out Tongue](http://www.ssnakess.com/forums/images/smilies/tongue.gif)
Eilonwyvern was the first to have her own litter. Then Bula became pregnant and would lie on top of all the babies! But when All of Eilonwyvern's babies were busy living in my freezer and Bula gave birth, Ellie at them ALL! (Well, Bula only had 3 babies anyways ... but Eillie at them and that's the point that I'm getting at!)