Ring groups are not an Asterisk concept, but rather an Asterisk GUI concept, so I can’t be sure exactly what you mean.
There is or was a bug which causes inappropriate use of the message you quote. The intended purpose is that when several numbers are being rung in parallel, Asterisk should not end up trying to ring the same number more than once, but the test can also trigger in other cirmcumstances.
I don’t know if it is fixed in the latest version, but I believe a patch has been added to the issue report on issues.asterisk.org/jira/