Doubling vocals tracks is a technique that has a bit of mystery to it. Many albums have double tracked vocals you wouldn't even notice while commercial releases often have much more than that. It's an art within itself and there are many different ways of going about it which will result in very different sounding results. To start off with this subject we found a good article from recordingreview.com which explains some of the different ways you can go about. You can check out the article here.