I now make a slight change to !CB-OK.PY by using a global variable (as per the randomHeadVerts.py sample script) as an incrementing callback counter to get this, which will increment the value of yRotate each time the callback is called:

And yes, modifying ANY parameter on the Body (or doing any of the other UI things*) causes yRotate to increase by 4, meaning that the callback IS called multiple times.

So I'm not going mad - at least no more than I already was.

So WHY does the SetUpdateCallback() cause this to happen ?

*Not true - selecting another body part and then reselecting Body appears to cause a single callback, and Edit > General Preferences > Cancel doesn't actually cause any callbacks, but going to the material room and back does cause four.

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The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) 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.