Resolved issue where waitgroup Done could have been called twice (resulting in an negative waitgroup count)

[daninmadison]

Because a previous loop could have resulted in the sync.Once wg.Done being called, my earlier commit this morning could have resulted in the wg.Done being called twice.

This corrects the issue by correctly using the sync.Once signal for any wg.Done inside the listen goroutine.

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This change is