App Store prompts me to install Security update 2020-006 10.13.6 multiple times. Why?

Yeah that's interesting, it used to be that if the App Store kept on

saying there was an update available even though you had installed it

a visit to the System Report would show it hadn't installed.

When you installed these updates did you monitor their progress,

there should have been a progress bar in the App Store Updates

page and then when it was ready to install the mac would have

shut down and restarted. As the mac restarted there should have

been a dialogue with an estimate of time remaining for installing the update.

I would download and install the standalone update as linked to above.

After installing that then check with the App Store to see if it is still being

listed as available for install.

If it is still being listed as being available then do the update in Safe Mode.

How to use safe mode on your Mac – Apple Support

Safe Mode takes a long time to boot up so be patient.

It may be prompting you to download and install the Security Update

multiple times because it has not actually installed it.

Click on the top left of your screen.

Click on About This Mac then click on System Report.

Go to Software> Installations, click on Install Date twice,

the list will now show the most recent installs first.

If it has not installed you can download the standalone update from here,

https://support.apple.com/kb/DL2059?viewlocale=en_US&locale=en_US

Once downloaded double-click to install.