Water Otter (clever name), you probably have one of two things:
1. Improperly installed/defective/whatever checkvalve. This will typically cause problems whether you are using water (faucet somewhere on) or not using water. With all faucets off, look at the pressure valve. If the system tends not to hold pressure, then probably you have a checkvalve problem, although it could be a leak somewhere but you would probably have already noticed that.
2. If the system will hold pressure with the faucets off, then turn on a faucet and watch the pressure valve. If everything is wonderful, the pressure should drop SLOWLY over a couple of minutes to the cut in pressure of 28#. If it is getting there (as I am going to predict) in just a few seconds, then your pressure tank has problems and is waterlogged. Assuming it is a bladder type tank, then replacement is the only long term solution.
To answer your direct question, the footvalve is a checkvalve and thus (as SB points out) the second check valve is redundant. However, I am not aware that a second check valve would be a problem. In fact, in deep well situations, it is advised in some quarters that a check valve be placed every 100 feet. So I would look at the tank. As you seem to have guessed, the fact that the system is holding prime would seem to indicate that the checkvalve situation is probably OK.
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