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I think it is just one of those things that works on paper, and not so good in reality. 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 would 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. 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.
