There is an update shown in the App Store for my Mac 10.13.6, but my mac already has this, what is the issue

Many people have had this problem, me included, it means it came close to finishing but did not.

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode?

When you go to the AppStore>Updates, what is the name of the update?

Then in System Report>Software>Installations, is it listed there?

Great news, take care my friend. :)