Rashi on Deuteronomy 15:5/רש"י על דברים ט״ו:ה׳

1רק אם שמוע תשמע ONLY IF THOU WILT CAREFULLY HEARKEN [UNTO THE VOICE OF THE LORD THY GOD] — then לא יהיה בך אביון THERE SHALL BE NO NEEDY AMONGST YOU.

2שמוע תשמע [IF] THOU WILT CAREFULLY HEARKEN — The repetition of the verbal form suggests: If one listens a little (if one shows the determination to obey) he is caused to listen to many things (he gradually becomes obedient to every divine command) (Sifrei Devarim 115:1; cf. Rashi on Deuteronomy 11:13).