Here's a recent case interpreting the Illinois Parentage Act touching on what happens when a Voluntary Acknowledgment of Paternity has been signed by two parents and later on one of them thinks it's a mistake...i.e. one of them claims usually the alleged father is not the father. The case is unusual because it's actually the child's mother who tries to vacate the court's order against the first father.

The case emphasizes again the huge stakes of acknowledging you're the father without DNA testing. If you're wrong and you don't get into court within 60 days you're likely stuck. You're on the hook for 18-23 years of various forms of child-related financial support.