quote from book titled Frequent Confession by Benedict Bauer:
"Now, once a sin has been committed it remains a fact always that sin has been committed , even after it has been fully pardoned. And it is possible for a person again and again to turn away interiorly from such a sin, to condemn it, to be sorry for it, to resolve to avoid it for the future and to amend his life, to do acts of penance for it.
There is no reason why such interior dispositions as they are present, should not be elevated by the power of Christ in the sacrament of Penance and made fruitful of grace.
For also in this case, where sins that have been already confessed and forgiven are confessed again, the sacrament produces its essential effects: it increases sanctifying grace, which from its very nature as the fruit of the sacrament of Penance, blots out sin if such happens to be on the soul.
The grace produced by the sacrament of Penance cannot be conceived without reference to sin: to the sin it would take away were the soul in a state of sin. Therefore, the words of the priest, “I absolve thee,” have their full meaning even when they do nothing but increase grace and do not actually take away sin for the simple reason that there is no sin there to take away.
Hence the Church teaches that sins that have already been confessed are “sufficient matter” for confession (Code of Canon Law 1917,canon 902)
And Pope Benedict XI in 1304 declared that it is “salutary” to confess sins that have been confessed previously."