 As for taking these two propositions to be distinct, there are many serviceable accounts, mostly involving some variation of the original proposal in modern philosophical semantics, found in (Frege 1892, 1970). According to Frege, every meaningful expression or phrase has both a customary reference that it denotes, and a customary sense that it expresses. In the case of clauses, the customary reference would be a truth-value which is compositionally derived from the denotations of the words of the clause, and the customary sense would be a way of thinking of that truth-value, compositionally derived from the senses of the words of the clause. Therefore, provided that ‘Superman’ and ‘Clark Kent’ have different though coreferential senses (i.e., provided they express different ways of thinking of the same individual) we will get different propositions. (However, it’s highly non-trivial to find an adequate account of the senses of names, given the critique of the most straightforward accounts in (Kripke 1972).)