Yes, on a couple of different occasions. The one that stands out in my mind is a situation where a man had two wives. The original wife was sponsorable, which isn't a problem. He had children with the second wife and one of the children was very ill. It was a Canadian-born child. I was in the unique position of having to request, on humanitarian grounds, that the second wife be permitted to stay in Canada for the benefit of the child.
I'll be frank with you, I don't think there's anything you can do about polygamy. It's legal in a lot of different countries. I don't think the immigration system really has the tools to be able to address anything to do with that situation. I've racked my brains over it personally because I'm not a fan of the practice myself, but I really don't think there's anything in the immigration system that can be done about it.