If I see the same Pokemon that I have caught before, should I catch it again? (and use up some Poke Balls)
The only way to evolve is through candies. you receive candies catching pokemon that you already have (and its evolution branch), hatching eggs or transfering them to the proffesor. so if you catch a lot, you can evolve the ones with highest CP
Also, you win XP to level up your trainer
If you ever want to evolve and train that type of Pokemon, then yes, you should be catching them all.