Full Idea
If (in the Platonic view) manyness was contained in humanness it could never be one as it is in Socrates, and if oneness was part of its definition then Socrates would be Plato and the nature couldn't be realised more than once.
Gist of Idea
If the form of 'human' contains 'many', Socrates isn't human; if it contains 'one', Socrates is Plato
Source
Thomas Aquinas (De Ente et Essentia (Being and Essence) [1267], p.100)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.100
A Reaction
I suppose the reply is that since we are trying to explain one-over-many, then this unusual combination of both manyness and oneness is precisely what distinguishes forms from other ideas.