For any semantic web site, RDFa or similar tags will be added to web pages multiple times for multiple reasons. Families of predicates can work together, and predicate relationships not used by a particular application are effectively invisible to it.
For example, a person looking at the web page, i.e. the visual markup, would not see the non-visual RDFa markup. Likewise, one app might use location-centric relationships and not care about accompanying information. Another app might look at the same page and grab only the bibliographic connections. Part of installing a semantic web application, then, is to initialize the pages with necessary markings. The surface area of a semantic web application is the complete set of tags that it reads and writes. Tags are added, modified, and deleted as the business evolves changes.
Non-interference is an important property.