A check valve is a critical component of a sump pump system, and its purpose is to prevent water from flowing back into the sump pit once it has been pumped out. Check valves are typically installed at the outlet of the sump pump, and they allow water to flow in only one direction.
Some sump pump systems may have two check valves installed - one at the outlet of the sump pit, and another at the inlet of the discharge pipe. Having two check valves can provide an extra level of protection against backflow, but it's important to note that if either check valve fails, then water will be able to flow back into the sump pit.
If you're considering installing a second check valve in your sump pump system, be sure to consult with a licensed plumber or other qualified professional to ensure that it's installed correctly and will function properly.