Enforce no conflicts in pluginManagement

Sometimes you inherit from a 3rd party parent which manages certain plugin versions.
In your own parent (inheriting from the 3rd party parent) you might manage the same plugins to different versions.
Often this is not done deliberately but rather for historical reason (for a concrete use case look at https://issues.apache.org/jira/browse/SLING-9307). It would be helpful to have a Maven enforcer rule which would enforce that all plugins are only managed once in the pom hierarchy. Optionally one should allow only downgrades or upgrades in the lower level poms.