Question: What is the ruling regarding the one who repents from a sin, then he returns to it again, so he repents again, then he returns to it (yet) again and so on. What is the ruling regarding his repentance?
Response: These repentances are all correct, because whenever someone commits a sin then repents to Allaah from (having committed) it and fulfills the conditions for repentance – then Allaah accepts his repentance. And if he falls into the sin again, then he should repent for a second time, and then a third time and a fourth time as Allaah (Subhaanahu wa Ta'aala) says:
{Say: "O 'Ibaadee (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah, verily, Allaah forgives all sins} – [Soorah az-Zumar, Aayah 53]
However, that which is important is the repentance must be sincere and truthful, and that he must be determined never to return to (committing) that sin (again): His repentance must not be an empty one – that he repents, yet in his heart he has the intention to return to that sin – such a repentance is not genuine.
However, if his repentance is genuine, and at the time of committing the sin he was determined never to return to (committing that) sin (again), then if he was to commit the (same) sin again, his first repentance will not be nullified, rather, his first repentance is correct, and whenever he commits a sin and repents, Allaah will forgive him.
Shaykh Ibn 'Uthaymeen
I'laam al-Mu'aasireen bi-Fataawa Ibn 'Uthaymeen – Page 315. fatwaonline.com