I'd say my answer is that no project is static and you can think of the existing project as similar in this time and place, but you could eventually go in a different direction with the evolution of your project so it's not so bad to duplicate for now.
In terms of learning, duplication isn't so bad either.
If you really do have a finite amount of functionality you are trying to build for a library or framework and the exact same thing already exists and is being maintained well, it is probably an opportunity to get involved in the project that already exists. And if you find you want to take things in a different direction at any point, you can get back to your own idea.
As an entrepreneur I'm actually quite a proponent of a sort of "first replicate, then expand" path in terms of getting into a new space. Replicate doesn't mean generic clone, but you can replicate an existing business model that works before being more "innovative" once you've achieved parity. I think the concept of "replicate then extend" could bode well for any project.
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