The identifying attribute as not the same as the identifier which OPA assigns to each entity instance and is unique.
The identifying attribute is just a useful way of displaying information in a decision report or (the debugger) which can then refer to the instance of e.g "the child" as "John" or "Mary" rather than "child #1" or "child #2". In this example you would expect the identifying attribute to be unique within a family but not necessarily within the whole session which may include several families.
The identifying attribute can either be an base attribute input directly by the user e.g. "the child's first name" or an inferred attribute e.g."the child's full name" where "the child's full name = the child's first name & " " & the child's middle name & " " & the child's last name". You could then write a rule to enforce uniqueness of the "the child's full name" but this is really independent of the attribute being the identifying attribute for the entity.
By default, when you create an entity, an attribute of the same text is created and set as the identifying attribute. However, you can manually change this in the Properties file.
See this section of the OPM User Guide: http://docs.oracle.com/html/E48373_01/toc.htm#Data%20model/Define_an_entity.htm#Give2