Mini keeps updating every time I reboot

I have a Late 2012 Mac Mini. Current installed OS is 10.10.5. I'm wanting to update to the latest El Capitan, but have an issue I feel I need to resolve before I do.

For the past few weeks, every time I reboot my mini, I get the Apple logo with a progress bar underneath. It slowly fills up to about 20%, then rapidly finishes. By the time the progress bar is *almost* finished, the desktop is already starting to appear on the screen.

To me, it appears that the update is not finishing for some reason. Otherwise, why do I see the same progress bar every time I boot?

(It's rigth after this point that the bar completes quickly.)

Any ideas how to find out what's going on and how to fix it?

Thanks in advance!