When one is concerned about an object identification so as to make objects recoverable via their identity, a system of UniqueIdentifiers is usually devised. Whether a collectable signed and numbered by the originator, or a file name in a filing system, or an object or method in an ObjectOriented system, or a pattern. Those who are concerned about OnceAndOnlyOnce will necessarily be found utilizing methods of identification which are unique.
Other more numerous reasons for UniqueIdentifiers can be found in:
Universal Identifiers Use GloballyUniqueIdentifiers (GUIDs) in Access and SQL Server When does a Unique Identifier not need to be used? ItDepends on Context: Also, there's the 'tag' URI protocol, which I first saw proposed to be used within AtomSyntax for handling unique ids:Related:
Contrast with GroupsAreNotUnique