As I was arguing about a month ago, things can be accessed from multiple URIs, and without analyzing representations, it is difficult to conclude that those URIs are indeed pointing to the same thing. Search engines have now recognized this issue, and would like servers to include a new
canonical link relation to inform them of a canonical URI. For search engines, this simplifies de-duping.
Here is the example from the Yahoo! search blog.
<link rel="canonical" href="http://www.example.com/products" />
This is similar to one of the solutions I suggested in my original post on resource identity.