Full Idea
Insofar as identity seems vague, it is provably mistaken. If it is vague whether x and y are identical (as in the Ship of Theseus), then x,y is definitely not the same as x,x, since the first pair is indeterminate and the second pair isn't.
Gist of Idea
It can't be indeterminate whether x and y are identical; if x,y is indeterminate, then it isn't x,x
Source
Nathan Salmon (Reference and Essence: seven appendices [2005], App I)
Book Reference
Salmon,Nathan: 'Reference and Essence (2nd ed)' [Prometheus 2005], p.243
A Reaction
[compressed; Gareth Evans 1978 made a similar point] This strikes me as begging the question in the Ship case, since we are shoehorning the new ship into either the slot for x or the slot for y, but that was what we couldn’t decide. No rough identity?