Read the lines of poetry. what was the constant knock, knock, knocking waking me before dawn? why did the poet repeat the word knock? to show that the speaker has been asleep to show that the speaker is expecting a visitor to show that there is no doorbell to show that the knocking is continuous

The repetition of the word knock, first of all, conveys a specific literary device which is called Alliteration.  Here is the definition of alliteration: It is the appearing of the same, exact letter or sound at the starting point of adjacent or closely linked words.  Secondly, the repetitious sound of knocking , conveys the knocking as continuous.  Hope that this answers your question.