[seo_butterflyth]

Google has endorsed use of breadcrumbs in several of their videos. They are a helpful device for website with complex structure or hierarchy such as eCommerce sites. They not only aid user navigation but also help search engines get a clearer picture about your site's structure. For example if you're using faceted navigation you could end up on a product page that has a long URL and ambiguous placement in the site's hierarchy. Breadcumbs can aid with this. Another neat thing is in the SERPs when you get little: Top Level > Main Category > Sub Category type links under your snippet.

I would use it at the top only, it's become a bit of a standard and people know where to expect it. Referncing it twice would be just a doubleup of the same set of links.