Clean-room design
Recreating a program without copying it, by making sure the people who write the new code never see the original.
The work is split between two teams with a legal wall in between. One team studies the original and writes a specification of what it does. The other team, which has never seen the original code, implements that specification from scratch. Every document that crosses the wall is vetted by lawyers and archived, so that in court you can show nothing was copied.
The textbook case is the IBM PC BIOS: Compaq and then Phoenix Technologies cloned it this way in the early 1980s, and Phoenix sold the result to the entire clone industry. The method protects against copyright claims but not patents — an idea invented twice is still patented once.