“The Kohen shall provide him atonement before G-d, and it shall be forgiven him for any of all the things he might do to incur guilt” (5:26)
The Seer of Lublin commented that after a person achieves forgiveness for a particular sin, he is then in a position to discover that there were other things he did to incur guilt.
Improper behavior serves as a barrier to our awareness of the nature of our actions. When we are forgiven for something, the barrier is lifted, and we are then able to see that there were other things we did that require rectification and atonement.
This is why teshuva is an ongoing process. The more we are forgiven, the more sensitive we are to our actions. In other words, teshuva begets teshuva.