You can definitely overdo it but I find that if you go lightly you can put many coats down. You do have to wait for it to dry however or that's when you start to get into the fibers of the paper and that's a mess. Also, it seems to help if you work dark to light. I've gone both ways but try not to overlap anything while it's wet or you can loose your edges.

One of the things I like about Watercolor is that it forces me to let go a bit. I tend to work really tight and while that's good for some things it would be nice to loosen up and let the flow happen. Half of watercolor is about working with the flow of the medium which you can't always perfectly control.