I have KB2976978 installed (install dated 18th June 2015). Today this update appeared again as an optional along with KB3065988. Both were installed successfully and PC restarted as requested. I then left things for 20 minutes and did another restart to ensure all files were placed correctly. Left again for 15 minutes and then I run disk clean + system file clean. All OK. I then rebooted for a final time and was presented with a 'Important Updates Are Available' message on the log in screen. Checking Windows Update shows KB2976978 once again available, even though it was successfully installed only an hour or so ago.
Sounds to me like some aberration on the part of Windows Update. I'll wait and see if it disappears from the list tomorrow.