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The dishwasher line into the garbage disposal is higher then the main discharge line from the garbage disposal, it sounds to me like there might be a small blockage in the main line (P trap possibly), try the following test:
Put the plug in he sink and then fill your sink with water, pull the plug out and take notice of how fast the sink empties, it should empty pretty quickly, if it is draining slowly then this would allow the disposal chamber to fill with water and back flow down the dishwasher line.
If this is the case (draining slowly) try removing the P trap and seeing if there are any blockages.
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