Make sure that the drain hose is connected correctly, and that there isn't a direct run down from the sink u-bend to the washing machine. This can allow some of the dirty water running down the sink to run down the drain hose into the washing machine. A drain hose that is connected to a sink u-bend should first go up at least 3 or 4 inches high (it may need tying up) before running down to the washing machine. Good luck
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