BSP: tasks seem to be executed duplicately and concurrently about mill HOT 3 OPEN

I opened discussion #2820 to address this

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All the above was tested on 0.11.4. I notice there's significant changes to BSP stuff in 0.11.5, but the issue does still reproduce in 0.11.5.

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I'd assume, that if the source and the resource request run in parallel, and as there is no previous cached result, both also need to evaluate the directory target (for the first time). Mill has no mechanism to detect and avoid concurrent execution of the same task. This is rarele needed, as the task dependency tree only runs each dependency once, even in parallel mode. But in case you run two Mill processes in parallel or BSP runs multiple requests in parallel, this can become an issue.

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