Constraint: URIs Identify a Single Resource
Assign distinct URIs to distinct resources.
In a nutshell the authors have made it clear that a URI should refer to a particular resource. And just a bit further on they point out that URI's can be aliases for a single resource:
Just as one might wish to refer to a person by different names (by full name, first name only, sports nickname, romantic nickname, and so forth), Web architecture allows the association of more than one URI with a resource. URIs that identify the same resource are called URI aliases. The section on URI aliases (§2.3.1) discusses some of the potential costs of creating multiple URIs for the same resource.
They even offer thoughts on the performance consequences of aliases.
Ya gotta love it... :-)