Full Idea
From sameness of meaning there does not follow sameness of thought expressed. A fact about the Morning Star may express something different from a fact about the Evening Star, as someone may regard one as true and the other false.
Clarification
Hesperus (the evening star) and Phosphorus (the morning star) turned out to be the planet Venus
Gist of Idea
I may regard a thought about Phosphorus as true, and the same thought about Hesperus as false
Source
Gottlob Frege (Function and Concept [1891], p.14)
Book Reference
Frege,Gottlob: 'Translations from the Writings of Gottlob Frege', ed/tr. Geach,P/Black,M [Blackwell 1980], p.29
A Reaction
This all gets clearer if we distinguish internalist and externalist theories of content. Why take sides on this? Why not just ask 'what is in the speaker's head?', 'what does the sentence mean in the community?', and 'what is the corresponding situation?'