Leveling up your Pokémon usually takes one or two Candies and some Stardust, while evolving your Pokémon takes anywhere from 12 to 400 Candies. But you’ll get better results if you evolve your Pokémon before leveling them up (this takes some holding out—it’s tempting to spend that Stardust), because evolved forms of Pokémon take their unevolved forms’ Candy and have higher all-around stats.
Is there any reason to keep multiple Pokémon of the same type?
There’s no real reason to keep multiple Pokémon of the same type—it’s best to transfer the weakest, or lowest-CP, Pokémon to Professor Willow for candies. The only reason to keep multiple same-type Pokémon is if they are your highest CP Pokémon and you want to win gym battles. I personally wouldn’t keep more than two or three of the same type of Pokémon, though, since CP level is not the only thing that matters when it comes to battle.