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Examine the fill valve, the mechanism that brings water into the tank, to check if it continues to flow water even after the tank has filled to its correct level. The float on the fill valve should rise and shut off the water supply when the tank is full. A faulty diaphragm or seal within the fill valve will prevent this shut-off, leading to water constantly flowing into the flush valve's overflow tube and into the bowl. Also, check if the refill tube from the fill valve is submerged below the water line, as this can cause siphoning.