If you are getting that condition your are experiencing what is known as "tool deflection" You are probably too high above the centerline with your tool. When you make a pass the wood is "deflecting" the duplicator backwards slightly.
Once you have everything on center you should be fine.
BTW the same condition can exist when you have a dull tool. Also, something else to check for; On the duplicator carraige there are 4 setscrews with retaining nuts on them. I have seen instances where one has backed out slightly. This will allow the carraige to pivot back and forth. This will cause the condition that you describe. Adjust the screws so they exert even pressure on the nylon gib glides. Elliminate the wobble and you will see an immediate diffence .
I think I'll open a shop and call it Jiffy-Dup
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