Firstly: 

If a person commits sin time after time, his sin is forgiven every time if he follows his sin with repentance – if his repentance each time is sincere. The evidence that it is permissible to repent time after time is the fact that those who apostatized from Islam at the time of Abu Bakr were brought back to Islam by Abu Bakr and he accepted that from them, although he knew that they had been kaafirs, then they entered Islam, then they went back to kufr, then they came back to Islam. All the Sahaabah accepted their repentance despite the fact that what the apostates had done was worse than what a sinning Muslim does. So the repentance of a sinning Muslim, even if he sins repeatedly, is more likely to be accepted than repentance on the part of a kaafir time after time.  

But what we are saying is subject to the condition that the first and subsequent cases of repentance be sincere and from the heart, not simply an outward show of repentance. 

What we are saying should not be understood to mean that we are encouraging sin and that it may be committed time after time, and that the Muslim should take the mercy of Allaah and the fact that Allaah accepts repentance as an excuse to commit sin. No, our intention is to encourage the sinner to repent time after time. We want to put the mind of the Muslim who wants to come back to Allaah at rest, and tell him that the gate of mercy is open, and that Allaah’s forgiveness is greater than his sin, so he should not despair of the mercy of Allaah and he should keep turning to Him. 

Al-Bukhaari (7507) and Muslim (2758) narrated from Abu Hurayrah that the Prophet (peace and blessings of Allaah be upon him) said that his Lord said: “My slave commits sin, then he says, ‘O Allaah, forgive me my sin.’ Then Allaah says, ‘My slave has committed a sin, but he knows that he has a Lord Who forgives sin and takes away sin. So I forgive My slave.’ Then he commits sin again, and says, ‘O Allaah, forgive me my sin.’ Then Allaah says, ‘My slave has committed a sin, but he knows that he has a Lord Who forgives sin and takes away sin. So I forgive My slave.’ Then he commits sin again, and says, ‘O Allaah, forgive me my sin.’ Then Allaah says, ‘My slave has committed a sin, but he knows that he has a Lord Who forgives sin and takes away sin. So I forgive My slave.’ …” 

Al-Haafiz Ibn Rajab al-Hanbali said: 

Ibn Abi Dunya narrated with his isnaad that ‘Ali said: “The best of you is every tempted one who repents (i.e., every time he is tempted by this world, he repents).” It was said, “What if he sins again?” He said, “He should ask Allaah for forgiveness and repent.” It was said, “What if he sins again?” He said, “He should ask Allaah for forgiveness and repent.” It was said, “What if he sins again?” He said, “He should ask Allaah for forgiveness and repent.” It was said, “For how long?” He said, “Until the Shaytaan is defeated.” 

Ibn Maajah narrated a marfoo’ report from Ibn Mas’ood which says: “The one who repents from sin is like the one who did not commit sin.” (Classed as hasan by al-Albaani in Saheeh Ibn Maajah, 3427). 

It was said to al-Hasan: Shouldn’t one of us feel too shy to ask his Lord to forgive his sin, then do it again, then ask for forgiveness, then do it again? He said: The Shaytaan wishes that you would have this attitude, so do not tire of asking for forgiveness. 

And it was narrated that he said: I do not think this is anything other than the attitude of the believers, i.e., every time a believer sins, he repents. 

‘Umar ibn ‘Abd al-‘Azeez said in his khutbah: O people, whoever commits sin, let him seek the forgiveness of Allah and repent. If he does it again, let him seek the forgiveness of Allah and repent, and if he does it again, let him seek the forgiveness of Allah and repent. For it is sin which hangs around a person’s neck, and doom comes from persisting in sin. 

What this means is that a person will inevitably commit the sins that it is decreed he will commit, as the Prophet (peace and blessings of Allaah be upon him) said: “Every son of Adam has his share of zina (adultery) written for him, and that will inevitably catch up with him.” (Narrated by Muslim, 2657). 

But Allaah has given people a way out from the sins they commit and sin is erased by repentance and seeking forgiveness. If they do that then they will get rid of the evil of sin, but if they persist in sin, then they will be doomed. 

Jaami’ al-‘Uloom wa’l-Hakam, 1/164-165. 

And Allaah knows best.