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Over flowing hot water cylinder

My hot water cylinder is over flowing onto the roof and then the water is flowing into the house. It's happening at night when we have two showers and and then rinse the dishes or if someone has a bath. We have previously had plumbers look at it (TWICE) The first time they replaced the over flow valve on the roof and adjusted the water pressure and temperature. The second time, they replaced the valve that's inside next to the cylinder and adjusted the water pressure again. I'd really like it to get sorted properly.