I would loop over the repeater once, gather all of the values into an array and then use the array.
A simple example:
$values = array();
while (have_rows($repeater) }
the_row();
$value = get_field('sub_field');
if (!in_array($value, $values) {
$values[] = $value;
}
}
// loop over the array and do something with each value.
