A couplet consists of two consecutive lines that typically rhyme and have the same meter. In "Sonnet 116," the couplet is:
“If this be error and upon me proved,
I never writ, nor no man ever loved.”
This is the final couplet of the sonnet. The lines you highlighted earlier, “Love alters not with his brief hours and weeks, But bears it out even to the edge of doom,” form a separate couplet as well. However, the exact couplet you referred to in your question, duplicating the same two lines, is a repetition and does not represent a different couplet.