Full Idea
Does a thing share the whole of its form, or a part of it? - What's to prevent the form as whole from being in each of the many? [Parmenides:] So it will be at the same time, as a whole, in things that are many; and thus it would be separate from itself.
Gist of Idea
If a Form exists completely in may things, then it is separated from itself
Source
Plato (Parmenides [c.366 BCE], 131a)
A Reaction
This is one of Parmenides' major objections to the standard theory of Foms. A Form is a single unified thing, existing in many place simultanously, which contradicts common sense. The best reply is Forms are patterns, not objects.
Book Reference
Plato: 'Parmenides', ed/tr. Gill,M.L./Ryan,P. [Hackett 1996], p.131