Strange overprinting display that doesn't appear on separations
Recently upgraded to new iMac and re-installed Adobe CC. Since then every high resolution pdf i view has a strange "ghosting" effect of trim marks etc which only shows when viewing overprint - they do not appear in the separations preview nor do they appear when printing.
This appears on pdfs created by others and it does not affect anyone else in my group so is clearly a display error of some kind - any comments?

