User can no longer add same name .xsd to a vdb (name same, location(s) different).

Results in:
java.lang.RuntimeException: javax.xml.bind.UnmarshalException

• with linked exception:
  [org.xml.sax.SAXParseException; lineNumber: 61; columnNumber: 168; Duplicate unique value [<name>] declared for identity constraint of element "vdb".]
  at org.teiid.core.designer.CoreModelerPlugin.toRuntimeException(CoreModelerPlugin.java:76)
  at org.teiid.core.designer.util.OperationUtil.perform(OperationUtil.java:50)
  at org.teiid.designer.vdb.VdbUtil.getVdbManifest(VdbUtil.java:178)
  at org.teiid.designer.vdb.VdbUtil.isPreviewVdb(VdbUtil.java:120)
  at org.teiid.designer.runtime.preview.PreviewManager.isPreviewVdb(PreviewManager.java:267)
  at org.teiid.designer.runtime.preview.PreviewManager.findPvdbs(PreviewManager.java:656)
  at org.teiid.designer.runtime.preview.PreviewManager.handleModelProjectOpened(PreviewManager.java:895)
  at org.teiid.designer.runtime.preview.PreviewManager.done(PreviewManager.java:466)
  at org.eclipse.core.internal.jobs.JobListeners$3.notify(JobListeners.java:39)
  at org.eclipse.core.internal.jobs.JobListeners.doNotify(JobListeners.java:112)
  at org.eclipse.core.internal.jobs.JobListeners.done(JobListeners.java:152)
  at org.eclipse.core.internal.jobs.JobManager.endJob(JobManager.java:647)
  at org.eclipse.core.internal.jobs.WorkerPool.endJob(WorkerPool.java:105)
  at org.eclipse.core.internal.jobs.Worker.run(Worker.java:70)
  Caused by: javax.xml.bind.UnmarshalException