We each have to find what works for us. For me, I'm roughly planned out several years in advance in what I'm going to be making, other than motivation (last couple of years have been hard on me) I don't have much problem in purchasing duplicates.
I do have a friend who is notorious for liking a fabric and forgetting she already bought it and then finding she already has multiple yardage cuts of the same print. I don't think we can ever really help her, is an impulse control and not a quilting issue
Part of me says that if you can afford it, isn't much of a big deal to get duplicates of things for (my personal price point) of $20. Other people have higher or lower price points.
I have another friend who is an older woman, I've been helping her with her stash for a few months now. We've mostly been sorting things into groups -- there is a huge pile now of UFOs and PiGS (projects in grocery sacks) and tons of fabric she bought like 20 years ago to make curtains. I bought a package of large (like 5x7) index cards, we will go through the UFOs and write an index card for each one. It will include the name of the project, what stage it is in, and what is required to complete (including an estimate of time), along with the priority/likelihood of actually being completed.
I think she is going to be shocked at the number of how many projects she has (at least 20), but that she can at least hold that many cards in her hands at once. She's pretty much in denial as it is, even when confronted with the items. If she deals with the actual items, she gets distracted and overwhelmed and nothing gets done and nothing gets put back. The idea is that she can prioritize the cards into what does she want to do first. Likewise, any new ideas will need to have a card made for that project and it put in the stack with it's priority.