Charizard and Rhyhorn
Poliwag and Ditto
These are the only two pairs of Pokemon with the exact same cry. Even though the cries of two other Pokemon may sound the same, they have slight differences in pitch. For example, Machop and Omanyte. They sound the same, but if you listen close enough there is a slight pitch difference.
Also, you might find this hard to believe, but in Gen I, there are only 37 completely different cries, despite 151 Pokemon in that generation.