Halliday & Matthiessen (2014: 268):
The nominal group functioning as Attribute construes a class of thing and is typically indefinite: it has either an adjective or a common noun as Head and, if appropriate, an indefinite article … It cannot be a proper noun or a pronoun since these do not construe classes. (Thus he is Charles Darwin would be interpreted as ‘identifying’; but if we say he is another Charles Darwin, the clause is ‘attributive’ and the proper name Charles Darwin has been re-construed as the common noun – the name of a class of people that are like Charles Darwin.)