Reference: I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen – Page 315
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:
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.