Here is a link to a video you might be interested in, this is a duplicator that uses a floating router to follow a template.
I am actually planning on building a duplicator for my wood lathe. I am thinking of using a similar concept to the link, but mine is going to be based more upon a design like a CNC router, but with no stepper motors (his design is a little large and I think it could be designed to take up much less space, and well be able to use the lathe in a normal configuration too). I plan on having three axises that allow the router to move up and down, left and right, and forward to backward on the lathe. This way you can do more than just duplicating with the router, it will also have various templates for the different axises. Depending on your need, you might only need two axises. For the linear motion of each axis, I plan on using ball bearings that will glide on small 'beams.' My whole contraptions will mount on the backside of the lathe, that way you can still use the lathe as normal with chisels and the tool rest. I wish I had a drawing to show my thoughs, but hopefully my description is sufficient enough.
Hopefully this helps, Christian.