I would imagine your dishwasher outlet is connected to the sink. I would at first try lifting the middle of the hose to a high point above the end connection to the sink. If this has little effect, you will need to fit a stand pipe with trap or alternatively connect directly to the waste pipe with a dishwasher/washing machine drain out kit. These have a one way valve fitted and will solve the problem.
Regards
Gary the plumber.
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