“return, yes, return.” This is the figure of speech polyptoton, in which the verb is doubled for emphasis. God wants to be very clear that the robe taken as a pledge on the loan will be given back by sundown so the poor person has something to sleep in. Note that the most important thing in the verse is not the wealthy person getting a return on what he has lent, but for him to act righteously before God. On the Day of Judgment, the rewards from God for living righteously will far outweigh any material thing that could be gained here on earth in this life.
[For more information on the figure polyptoton and the way it is translated, see commentary on Genesis 2:16.]