The double Check Valve features two such units mounted in series with the water flow path. The concept behind this design is a two-fold combination of redundancy and efficiency.
1 Examine the area around the Check Valve and watch to see if water is coming from somewhere else and gathering on the Check Valve. Sometimes water can trickle down the water pipe and drip from the Check Valve, making it appear that the valve is leaking.
2 Wipe any moisture off of the Shower Drain