I think it is just one of those things that works on paper, and not so good in reality. A check valve is designed to stop water from going backwards. You don't want water to go backwards when the pump shuts off, so if one check valve is a good thing, then a hundred check valves should be better.
However, the more check valves you have the more friction loss you have. A bigger pump is needed if you have much friction loss. Other than that, you would still think an extra check valve or two would still be a good idea for backup.
Many times, multiple check valves do work OK when everything is new and working perfectly. There are only two kinds of valves in existence, those that leak and those that will leak. The problem comes when any check valve leaks a little bit, or doesn't close quite as fast as the check valve above it. Then a negative pressure is created below the top check valve.
If the top check valve is above ground, the vacuum can draw contaminates into the line. No matter if the top check valve is above ground or in the drop pipe, the negative pressure will allow the pump to start to the right of it's curve. (In other words "wide open") The pressure above the pump will change from negative pressure to positive pressure in a fraction of a second. The effect is, the column of water below the top check will "punch" the top check. This is much like a Boxer "punching" you in the face. It creates a shock wave that travels throughout the water lines. At every outlet valve, elbow, tee, and dead end in the pipe, the shock wave turns into water hammer. Water hammer causes everything in the pipe system to experience pressure 10 times what you see on the gauge. On larger pumps it usually causes the pressure switch to bounce on and off like a telegraph.
You can remove the upper check valve when they start causing problems, or you can install only one check valve at the pump and not have the problems.
I have installed systems as deep as 2200' with only one check valve. You just need a good check valve, and to start and stop the pump at "almost" deadhead pressure conditions.
The only check valve you should have on a submersible pump system is the one on the pump itself. Another check valve anywhere else can cause a vacuum or negative pressure before the second check valve. If that second check valve is above ground, the potential of drawing in contaminates because of the vacuum conditions is probable. If the second check valve is below the pitless, on anywhere on the drop pipe, the potential of a water hammer event happening at each pump start is very real.
Using a second check valve just in case the one on the pump fails is never a good idea. If the check on the pump fails, it must be replaced. Having a second check valve will only mask the problem of the first check valve, and cause water hammer and possible contamination in the process.
Cycling the pump on and off is what destroys check valves and most other equipment in a pump system. The check valve slamming shut from a full open position when the pump is cycling repeatedly is the main cause of check valve failure. Using a Cycle Stop Valve on your pump system dramatically reduces the number of times your pump cycles, which will increase the life of the check valve and all other components in the system. The Cycle Stop Valve is also in the 1 GPM position, not fully open, when the pump shuts off. This means that the check valve is also only open to 1 GPM, which is only the width of a piece of paper, not fully open, when the pump shuts off. This means that the few times that the Cycle Stop Valve allows the pump to cycle, the check valve does a nice gentle close, which eliminates water hammer and check valve failure.
Multiple check valves work good on paper but, cause multiple problems in the real world.
