I would argue that in many ways (that vary from project to project) the duplicated efforts are a good thing overall. Now, instead of having one single group or person working on a particular topic, there are many and the knowledge, learning, experience, and expertise that comes from that isn't locked up with only a select few. That can be applied in more areas, used for more things, or carried into other endeavors. It also hedges against all that expertise being lost if the few experts of the original project abandon it for whatever reason.

I don't see all the efforts that go into *BSD, Windows, macOS, etc... as being wasted just because Linux is available, nor would I consider the effort to try something completely new in os/kernel design that starts from a clean slate a waste, even if it doesn't end up becoming wildly popular. Not everyone who has the capacity to work on a duplicate effort would be able to contribute to the original, and the original will never be able to perfectly meet the goals/needs of everyone.