The repetition of the question lends a sense of urgency. The watchman probably refers to the prophet Isaiah, as in Isaiah 21:6. Asking how long the night would last was a symbolic way of asking how long it would be until the end of the oppression. The fact that Isaiah was asked this question acknowledges that he represented the true God.
11 The oracle concerning Dumah. One is calling to me from Seir, “Watchman, what time of the night? Watchman, what time of the night?”