Currently, any duplicate Pokemon with a low combat score I find, I'll send back to the professor for candies to evolve my high ranking ones.
Yet I wonder if I should evolve some of them first with candies and sent them back then. Is there a bonus of sending back evolved pokemon in regards of candy? Or would it be a waste of resources?
There is no bonus for sending back evolved pokemon as of now so you should just send them back. However I would still send them back if the have low CP or if you simply don't need them anymore.
However the only exception is if you save a ton of weak pokemon and obtain a lot of candies so you can do a lot of evolutions while you have a lucky egg on which will grant you a lot of trainer experience since all evolutions give you at least 500 experience. You get more if the evolved pokemon isn't one you have in your pokedex yet.
Source: Personal experience
