When you start seeing duplicates that's a good thing because your odds of getting purple (epic) and gold (legendary) is much greater and events are better.
As you climb levels, you can only unlock so much. I've barely gotten anything outside of the event from boxes since it started, since I have nearly all of the base items. It's probably beginning to get that way for you
Love duplicates .. I often want only 1-2 things but I always get 4 shit sprays .. Its often better to have duplicates for coins and then I can buy what I want.