I actually just got rid of some of the Pidgeys/Rattatas etc, I think a Spearow, too.
I like to keep the Raticates as they have different moves between them, and they're quite good throwaway Pokemon for taking on gyms.
(some Rattata I catch are like 120CP already, and they're so common that I easily have the candies to evolve them into pretty powerful sidekicks.)
The Koffing are the same CP but one has Sludge Bomb and the other has Dark Pulse, and I find it hard knowing which to transfer, as Dark Pulse would be handy against any Psychic types it may run into! But it's a pretty low CP, so I'll probably end up catching a higher one and transferring both I have currently, anyway.
The Pidgeotto is going to be a Pidgeot soon (I have like 30 Pidgey candies), and it will buddy up nicely with my other Pidgeot and form a formidable duo!