Full Idea
All identity is necessary, although there can be contingently true identity statements - those that contain non-rigid designators.
Clarification
'Rigid designators' baptise things across all possible worlds (e.g. Aristotle, water) [Idea 4949]
Gist of Idea
All identity is necessary, though identity statements can be contingently true
Source
Colin McGinn (Logical Properties [2000], Ch.1 n5)
Book Reference
McGinn,Colin: 'Logical Properties' [OUP 2003], p.3
A Reaction
A nice case of the need to keep epistemology and ontology separate. An example might be 'The Prime Minister wears a wig', where 'Prime Minister' may not be a rigid designator. 'Winston wears a wig' will be necessary, if true (which it wasn't).