During confession, if I mention (in passing or in reference to) a sin I've committed in the distant past, the priest usually asks if I had confessed it before. If I say yes, he says it has been absolved and I don't have to confess it again. What is the difference between a general confession and a regular confession if past sins that have been absolved need not be confessed again?
I assume that by "general confession" you mean privately confessing your sins to a priest, but in a way that is general and covers a large portion of your life history. It is important that the priest understands that you are making a general confession and agrees to hear your general confession. Priests have time constraints that often prevent them from hearing general confessions, and usually a special meeting needs to be scheduled for a general confession.
General confessions are not a matter of necessity. Most people never make a general confession. The purpose is therefore different from ordinary confession, for ordinary confession primarily exists for the absolution of mortal sins. Here are some of the reasons why someone might want to make a general confession:
Why Make a General Confession?
The idea behind making a general confession is simple and profound.
Remember, the sacrament of reconciliation is meant to be a significant encounter with God’s mercy. At particularly poignant moments in our faith journey (like those mentioned above), preparing a general confession gives us a good opportunity to prayerfully reflect on our whole life history, and on how faithful God has been to us throughout that history even when we weren’t so faithful to him. Going over our whole past, or a significant chunk of that past, together with the Lord, is meant to bring us to a new appreciation of our need for God, of the abundance of his mercy, and of the depth of his care for us.
Another benefit sometimes accrues to this devotional practice as well. When we patiently take time to review all the sins of our past life in the presence of God, the Holy Spirit will often enlighten us regarding not only individual falls, but regarding patterns of sin and underlying attitudes that make us vulnerable to temptation. These insights can serve as a valuable guide as we prudently identify a path of spiritual growth for the future. In other words, making a general confession can be an effective way to grow in self-knowledge, such an essential element for spiritual progress.