This has been a huge worry of mine as well. I've thought of a possible solution, though I'm open to hearing criticism about it.
What if, since the cosmic catsack is the rarest of all, why not let people choose the final reward from it? Hear me out. This way, you can get a dupe of a particular item if you want it, but collecting every cosmic catsack item is still possible (and much more feasible) since you can control what you get. Obtaining a cosmic catsack in the first place already has astronomically low odds; Getting an item you actually want out of a cosmic catsack out of a small group of items would reduce that chance even further. Assuming there's going to be fewer, but higher rarity items in cosmics, I don't see it as being much of a problem letting the player choose from a few arbitrary, but rare rewards. What are your thoughts?
Here's a UI mockup I made in about 15 seconds in MS paint to show what it might look like when you trigger the prompt to open it.