I don't actually know how often inadvertent copying of other artists' work occurs. But I've found that copying someone's personal style is incredibly difficult to do. I've tried to copy a few styles, just for a lark and to see if I could, and have met with practically no success. I could be totally and completely wrong, but I would think that unless you spend a lot of time tracing one artist's work, it'd be hard to end up copying every element of his or her style. And if the style that you develop happens to share an element here and there with others' styles I don't think it's a big deal. That's bound to happen anyway. There are only so many ways to draw a schnoz after all. ;)

And as far as actual development of personal style goes, I've no clue how it really happens. I can only speak from personal experience, but I've drawn basically the same way since I was in middle school. The only difference is that in recent years I've tried to improve my skill.

I guess what I mean to say is don't be afraid to use others' work as anatomical reference. If you find that you are starting to imitate someone else's style then you'll know that it's not a viable resource. But I don't think that will happen. ;)