The game is based on an RNG system. It is not uncommon to get the same drops from engrams. I currently have 3 of the same exotic chest pieces and a couple exotic guns that are duplicates. It's not a huge loss though because once you get to higher light levels you can either infuse other guns into them (to raise their power level) or you can destroy them for components and rep from the gunsmith. If this bothers you, Destiny 2 is not the game for you. Read up on how to get set in-stone gear and how you can minimize the RNG elements as much as possible.