Water backs up in a double kitchen sink because both sink basins share a common sanitary tee fitting and a single P-trap underneath, meaning any blockage located in the shared branch pipe or trap forces wastewater traveling down one drain to exit upward out of the opposite basin instead. Additionally, congealed grease and food particles frequently accumulate right at the junction where the two tailpieces meet. Verification: Check the shared T-junction and P-trap beneath the basins for trapped food debris or blockages.
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