How does the repetition in "Tonight I can Write" by Pablo Neruda and The Love Song of J. Alfred Prufrock by T. S. Eliot affect the overall meaning of the respective poems?
The repetition in this poems is of differing styles therefore serves different purposes and has different effects. Simply put, the effect of repetition on Neruda's poem is that it produces a hollow echo like his hollow echoing heart. The effect of repetition in Eliot's, in contrast, is that it produces a contemplative and self-accusatory introspection.
In Neruda's, the poet focuses on his experience of a world devoid of his beloved's presence:
... 'The night is starry
and the stars are blue and shiver in the distance.'
The night is starry and she is not with me.
In Eliot's, the poet focuses on the psychological impact he has upon those around him and the psychological impact time has upon him:
And indeed there will be time
To wonder, "Do I dare?" and, "Do I dare?"
[They will say: "How his hair is growing thin!"]
The overall meaning of Nerda's is that there exist a contemplation of missing and suffering: though one suffers, one misses--the universe misses--the one whom one loved (or thought one loved) even while one contemplates yet no longer wants.
my soul is not satisfied that it has lost her.
Though this be the last pain that she makes me suffer
The repetition affects the meaning of Neruda's by creating an echo going through the poet, through the universe, through the night, an echo created by her absence and by the void ironically left by the absence of suffering.
The overall meaning of Eliot's is that there is felt, there is known, a psychological contrast between the dream and the reality, between the illusion and the reality. This is the idea expressed throughout by the accumulating contrasts, and it is the idea expressed by the contrasts in the resolution to Prufrock's narrative:
I grow old . . . I grow old . . . 120
I shall wear the bottoms of my trousers rolled.
We have lingered in the chambers of the sea
By [mermaids] wreathed with seaweed red and brown 130
Till human voices wake us, and we drown.
The repetition affects the meaning by creating a psychological pulse and tension that throbs behind the accumulating contrasts so that they are psychologically felt as well as intellectually perceived.