The speaker of this poem utilizes repetition from the first stanza; the final line of that stanza implores his father to "rage, rage" against the process of dying. Repeating a word in quick succession is a literary technique called epizeuxis. The effect of this type of repetition is to establish a vehement sense of urgency. "Rage" is a particularly strong verb to use here; it connotes a sense of violence and anger. Thus, the speaker begs his father to bolster up the energy for a battle against death, refusing to simply slip gently away from life.
The final word in each stanza also exemplifies repetition, alternating between night and light. Night symbolizes death in this poem, and light represents life. Through this alternating repetition, life and death are inextricably linked to each other. The road of life must inevitably lead to death; however, the speaker issues a repeated command, saying, "Do not go gentle into that good night," beseeching his father to continue living. Though the speaker realizes that a metaphorical night must eventually come, he asks his father to hold on to the light for as long as possible.
The repetition of these symbols and phrases convey the depth of the speaker's love for his father. Unwilling to imagine a world without his father in it, the speaker is even willing to accept the "curse" of his father—as long as it conjures up enough strength for him to fight fiercely. The repetition helps to establish a tone of intimacy as the speaker begs his father not to leave him.