Should we allow a single Runnable instance to run multiple times concurrently?

[randomir]

Since Runnable.run() is (via dispatch()) offloading next() to an executor, it's possible to run a single instance's next multiple times in parallel.

This is not a problem if next() does not mutate the self, but it could potentially lead to a state lost, or even a deadlock if it does.

Should be categorically ban such a behaviour, or leave the responsibility to the block/workflow developer?