Alright, you said it was a problem that is thought impossible, but needs to be on the list of possibles...I'll go with my wife's suggestion...double exposure. Hey, I'm a believer in Occam's Razor.
Yay!!! We have a winner.
Tell Mrs ToxMan that she wins a cigar.
I don't usually use the IR filter with the SCT but it was already screwed on after use on my Carton refractor.
I did a dust check and all was fine so I left it in place.
Possibility No 1:
What I didn't realise was that the filter had become loose on the thread and may have tilted just enough to cause a ghost image from the two faces of the filter.
Possibility No 2:
A "narcissus" effect caused by reflected light from the CCD returned by the filter.
Possibility No 3:
If (2) Then this could also be caused by a slightly tilted ccd.
This also begs the question - does the use of any filter cause a similar effect when imaging bright objects?
Even with a perfectly square filter there must be some overlay of a ghost image.
In my work (motion picture lenses) when front filtering this effect happens when using multiple filters. Our filter stages can tilt to remove this problem...
No wonder the stacking went bonkers.