I expect whatever message we use will become a Stack Overflow question in fairly short order, so "good enough" is almost certainly good enough here, and I'm massively overthinking the problem :)

However, combining my last suggestion with a variant of Martin's would give us:

    "Cannot reuse already awaited coroutine"

The generic "reuse" both eliminated the repetition of "await", and also gets us away from worrying about exactly what the caller was doing.