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I don't know if it's coincidence or not, but I found that when opening a lot of coffers one after the other, I tend to get a lot of the same item. Maybe it's just rng paranoia, but you could try opening them with little breaks in between or on different characters.
before the collection book i always just tried to have at least 1 of each monster shoulder for use if i wanted it, i fill in the remainders with gold vendor when they pop up there
doing this method ive actually not spent keys on anything other than the undaunted outfit pages for almost 2 years and have large majority of monster shoulders collected
the gold vendor monster stuff is especially good now that it is a 1 time buy to unlock things instead of needing to repeat buy if you wanted on multiple characters