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Full Idea
A simple extensional semantics will assign the same semantic value to 'Roger Federer' and 'world's best tennis player', but they clearly differ in meaning, and if events had unfolded differently they would pick out different people.
Gist of Idea
'Federer' and 'best tennis player' can't mean the same, despite having the same extension
Source
Laura Schroeter (Two-Dimensional Semantics [2010], 1.1.1)
Book Ref
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.3
A Reaction
You would think that this would be too obvious to need pointing out, but it is clearly a view that had a lot of popularity before the arrival of possible worlds.