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Most likely the the front shifter is pushing or pulling the chain too far.
Have some one grab the seat and hold the rear wheel off the ground while while you pedal and shift between the front chain rings. You will see that a arched chain guide pushes the chain in or out. If it goes too far it will knock the chain off. There is usually a small stop screw that will limit the travel of the shift cage which you can adjust to correct it. At least that is the way it worked 40 years ago when I worked on them.
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