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At higher levels, you may want to keep multiples in order to have different movesets but in a few levels you will be trading those for higher CP versions anyway.
I actually just got rid of all of my duplicate pokemon today by transferring them. I toyed with the idea of keeping one of the lower evolution pokemon (for example, pidgey) when I already had pidgeotto and pidgeot. Ultimately, I decided to transfer any pokemon that I already had a higher for of evolution. This got me a few extra candies to use to power up my highest evolved pokemon for that species.