It really has nothing to do with an encoder. Any CNC machine that can follow a contour with a probe, 2D or 3D, can also simultaneously, drag a spindle along the same path. If it is done in real time, the probe stylus and the cutter have to be the same size to make a duplicate part. This assumes a single set of 2 axis slides for 2D, ie a single machine. A pantograph type of fixture could also be used for scaling, but NOT for cutter size compensation.
If the probing and machining are decoupled ( ie through a cad model and g-code) the duplicated part can be scaled, mirrored, modified, smoothed,etc.
There are usually limitations on the response time of a probe, that may make it possible to machine the part faster than it can be probed. Parallel port controllers have definite bandwidth issues with probing speed. That will probably be the limiting factor. For a serial port, dedicated controller like DeskCNC ( 125,000 sps), the physical switch opening and closing becomes the limiting variable. The DeskCNC probe can physically detect around 20,000 to 30,000 points per hour.
Fred Smith - IMService