When building a plain Java environment, we create our own temporary extension registry by pulling in all the plugin.xml files that we can find on the build path.  If there are multiple identical jars or different versions of the jar, then the amalgamated extension registry will have duplicate entries that will cause the above error.  As a rule we only ACTUALLY load the latest version we find, but because of the duplicate components, the above error is thrown.  To resolve the error message, remove the duplicate jars from the path, but unless you want the older version of the jar to be used during the build it can be ignored.