The answer to 'backflow preventer overflowing' is that a backflow preventer that is overflowing is likely indicating a problem with the device or the plumbing system. Backflow preventers are installed to stop contaminated water from flowing back into the clean water supply, so an overflow is a serious issue that needs to be addressed.
Common causes of a backflow preventer overflowing include a clogged or malfunctioning backflow preventer, a problem with the plumbing system, or high water pressure. Symptoms may include water leaking or spilling out of the backflow preventer, foul odors, or discolored water. Solutions can include cleaning or replacing the backflow preventer, adjusting the water pressure, or addressing issues with the plumbing. If you are not comfortable troubleshooting and repairing the backflow preventer yourself, it's best to call a licensed plumber to diagnose and fix the problem.