A double elimination heat bracket involves having two paths to the final: through the QUALIFYING DRAW, where you advance in all heats to the final, and through the REPECHAGE DRAW, where you are eliminated from the main draw, and have to compete against all the eliminated athletes from each round. This format is commonly used in ISA events in surfing.
Example of a 16 person double elimination:
These formats can be built using LiveHeats draw editor, and the below video describes how.