The conversation that is repeated over and over in the novel is the one about the dream that George and Lennie share. In their dream, Lennie and George see themselves owning their own farm and living off of the land.
This conversation is repeated multiple times, mainly at Lennie's request, because he is mentally decelerated and forgets things consistently. He also needs to hear the words exactly how George says them to reassure himself that this dream could possibly come true.
When Lennie requests that George narrates their dream, it is acknowledged that this conversation has happened many times before:
Lennie spoke craftily, “Tell me— like you done before.” “Tell you what?” “About the rabbits.”
This constant repetition upsets George, to a point, but he always ends up repeating the narrative anyway.
George's voice became deeper. He repeated his words rhythmically as though he had said them many times before.
Other evidence that shows that the conversation has happened many times...
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