From an ancient (1974) book called "The Elements of Programming Style": "Make sure your code does 'nothing' gracefully."
Fast-forward to 2016.
After several tries, the Virtualmin GPL install.sh script finally ran without errors.
Then, absent-mindedly, I ran it yet again.
INFO - Download of http://software.virtualmin.com/gpl/rhel/7.2.1511/x86_64/virtualmin-release-latest.noarch.rpm Succeeded.
package virtualmin-release-1.0-5.gpl.rhel.noarch is already installed
FATAL - Fatal Error Occurred: Installation of virtualmin-release failed: 1
FATAL - Cannot continue installation.
FATAL - Attempting to remove virtualmin repository configuration, so the installation can be
FATAL - re-attempted after any problems have been resolved.