I am running 10.13.6 (installed over 10.13.6 ß6). Since the beginning of October, Software Update has prompted me to install Security Update 2018-002 three times - on Oct 4th, Oct 10th, and this morning (Oct 18th). I've done so each time and haven't observed any obvious harm from what appears to be repeated re-installation of the same software update.
BUT, this gets me wondering whether this is correct or incorrect behavior on the part of Software Update and what I can do to prevent it from happening (unnecessarily) again.
Note: the build number that is reported via About This Mac is changing. Before this morning's update, it was 17G3015, now it is 17G3020. Should I interpret this to mean that I am receiving updates (betas?) of a Security Update?
The answer is yes, you were installing Security Update betas and indeed there should be the word "Beta" in the update details.
Now you should be prompted with the first beta of Security Update 2018-003.