The Mabit, in his discussion of the status of a sinner’s repentance if he subsequently repeats the offense (Beit Elokim - Sha’ar Ha-teshuva, chapter 6), cites the Gemara’s ruling in Masekhet Yoma (86b) regarding confession on Yom Kippur for sins of prior years. According to the majority ruling among the Tanna’im, one should confess on Yom Kippur only for sins committed during the previous year. Assuming one had confessed his misdeeds of previous years during previous observances of Yom Kippur, he should not confess them again. The exception, however, is a sinner who confessed his sin last Yom Kippur but has since repeated the misdeed. In such a case, the individual must, indeed, confess even for the prior instances of the sin in question.
Seemingly, the Mabit observes, this ruling proves that repeating a sin after repentance retroactively renders the repentance meaningless. If the repetition of a sin requires one to repent anew for the past instances of that sin, we should, apparently, conclude that the initial repentance retroactively loses its value, as the individual repeated the offense and thus showed that his resolve to change was insincere or otherwise deficient.
The Mabit dismisses this argument, advancing a different explanation of the Gemara’s ruling. The Mabit writes that the Gemara requires repenting for the previous sin not because the repentance was retroactively deemed valueless, but rather to strengthen the individual’s resolve in the present. The Gemara instructs the sinner in this case to verbally recall his having earlier transgressed this violation and repented, so that he will make a greater effort than he had made before to avoid the sin in the future. Although his repentance was valid, as he genuinely regretted the sin and resolved to avoid it in the future, he clearly needs to work harder to improve, and Halakha thus requires him to reflect upon his entire history with this misdeed so he will recognize the need to invest greater effort this time.
A different approach is taken by Rav David Levanon, in his Be-yerach Ha-eitanim (pp. 47-48). He writes that if a penitent sinner repeats his offense after repenting, then he must repent for the initial sin because its recurrence may perhaps reflect a fundamental deficiency in his repentance. The recurrence does not prove that his repentance was not wholehearted and was thus valueless, but this possibility certainly exists. As such, he must repent again for the initial sinful act in case his repentance is retroactively deemed invalid.
Another explanation might be suggested by distinguishing between different levels of repentance. Rav Soloveitchik (Al Ha-teshuva, pp. 15-33) famously distinguished between “atonement” and “purity,” the former referring to a pardon, and the letter referring to the elimination of all effects of sin. The lower level of repentance achieves atonement, such that the penitent sinner escapes punishment for his wrongdoing, whereas the higher level achieves a state of purity, whereby the individual is cleansed of all adverse effects of his misdeed. Accordingly, we might suggest that if a person repented for his sin and then committed the act subsequently, although his repentance remains valid, and he had earned atonement, he clearly has failed to purify himself. The sin’s recurrence indicates that the individual has yet to rid himself of the sin’s effects, despite his having achieved atonement, and that he has not undergone the full transformation that the highest level of teshuva entails. Therefore, he must repent anew even for the initial violation, for which he had already earned atonement, in an effort to cleanse and purify himself from the sin’s adverse effects on his being.