In multi module builds same artifact id can be repeated with different group id. For example if project strategy is to use nesting of modules, instead of using long artifact names, ap or model module can occur multiple times with different group id.
By default maven, upon failed build, prints out only -rf :artifactId. When developer will copy given suggestion and run build again then his build will be started from first module which have given artifact id.
While severity of this bug is quite low, it affects productivity in bigger projects big time. To improve usability of returned message a Maven should return -rf groupId:artifactId once artifact id is not unique.