It'd be perfectly valid to mark up the same content in multiple locations. The point of microdata is that it makes your information understandable to machines (search engines, browsers, applications, what have you), so wherever data occurs on your site that you can mark up with microdata, you should mark it up.
Generally speaking, I would recommend using as much microdata as you can. While it isn't doing much more than power Rich Snippets right now, its applications will grow in the future. At the very least, it's worth structuring your code in such a way that you can add it later on.
The only thing I'd be careful of is the duplication of information itself. Small amounts of standard "boilerplate" information like an address and phone number should be fine, but anything more substantial that should be carefully considered. If you're feeling the need to reproduce large amounts of information across multiple pages, there's probably some scope to improve the site's architecture.