From my own experience and seeing a few people post, never decrypt more than 1 exotic engram per visit to the crypturd. Decrypt, walk away for a minute or two. There isn't any hidden timer or anything, but results from the field indicate doing multiples results in increased chance for duplicates.

For which character? Titan I presume?

Its just like the loot cooldown on chests. Wait at least 5 minutes between turning in engrams or you'll likely get repeats. Artificially inflating the grind is bungies primary skillset

If the game never gave repeats, I would have gotten every exotic in game by now, even the ones for the other classes. Currently, I am still missing a ton. I feel like Bungie made the drops for exotics far higher than D1, but because of that, they give us more duplicates. I have gotten 5+ of certain weapons or armor pieces. They should probably tone down the drop rate slightly, and make it slightly more likely that you get exotics you don't have.

Destiny 2 uses an mmo-style structure. That's just how mmos work. Take it from me, a long time mmo player, the rng in this game is a lot fairer than other games of this type I've played. Besides, what if you didn't pick the graviton lance and realised that it was actually more your taste than the exotic you picked? I took the graviton lance and haven't gotten either of the two other options I could have had yet so it would make sense to offer these items which were in high demand to start with so people who didn't get them don't feel like they're at a disadvantage. You getting loads of duplicates is just really bad luck, it happens. You'll get something you really like eventually and it'll feel really good. That's how this type of game works.