Well, it's definitely something to do with putting those three lines in the callback that causes all the oddities (multiple apparent calls of the callback, and the callback apparently being called multiple times if ANY parameter ON THE BODY actor is changed (or even if an unrelated UI action is performed such as "Edit > General Preferences > Cancel" )

These three lines:

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ac = poser.Scene().ActorByInternalName("BODY")
yrotval=ac.Parameter("yRotate").Value()
ac.SetParameter("yRotate",yrotval+1)

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I added those three lines to the 'parmCallback.py' sample callback and I got the same weird effects that I've noted

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.

*also available in ShareCG, DAZ, and HiveWire3D flavours (the DeviantArt and CGBytes flavour have been discontinued).