Common problem. The check valve on your water pump is leaking, allowing city water to backflow through the pump into the fresh water tank.
Turn off city water. Turn on water pump. Open a faucet and allow the pump to pump water for a minute or so. This will flush out the check valve. Close faucet, turn off pump, turn city water back on.
This usually solves the problem. If not, the check valve may be damaged. Fix then is to either replace the check valve (the valve is usually internal to the pump) or add an external check valve.
See attached generic plumbing pic.
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