Build path contains duplicate entry: 'org.eclipse.jdt.launching.JRE_CONTAINER' for project 'rss15.nsf'

Java Model Exception: Java Model Status [Build path contains duplicate entry: 'org.eclipse.jdt.launching.JRE_CONTAINER' for project 'rss15.nsf']
    at org.eclipse.jdt.internal.core.JavaModelOperation.runOperation(Unknown Source)
    at org.eclipse.jdt.internal.core.JavaProject.setRawClasspath(Unknown Source)
    at org.eclipse.jdt.internal.core.JavaProject.setRawClasspath(Unknown Source)
    at com.ibm.designer.domino.ide.resources.internal.builders.BuildPropertiesBuilder.checkIfRecreateClasspath(Unknown Source)
    at com.ibm.designer.domino.ide.resources.internal.builders.BuildPropertiesBuilder.generateDescriptor(Unknown Source)
    at com.ibm.designer.ide.generators.AbstractDescriptorBuilder.fullBuild(Unknown Source)
    at com.ibm.designer.ide.generators.AbstractDescriptorBuilder.build(Unknown Source)
    at org.eclipse.core.internal.events.BuildManager$2.run(Unknown Source)
    at org.eclipse.core.runtime.SafeRunner.run(Unknown Source)
    at org.eclipse.core.internal.events.BuildManager.basicBuild(Unknown Source)
    at org.eclipse.core.internal.events.BuildManager.basicBuild(Unknown Source)
    at org.eclipse.core.internal.events.BuildManager$1.run(Unknown Source)
    at org.eclipse.core.runtime.SafeRunner.run(Unknown Source)
    at org.eclipse.core.internal.events.BuildManager.basicBuild(Unknown Source)
    at org.eclipse.core.internal.events.BuildManager.basicBuild(Unknown Source)
    at org.eclipse.core.internal.events.BuildManager.build(Unknown Source)
    at org.eclipse.core.internal.resources.Project.internalBuild(Unknown Source)
    at org.eclipse.core.internal.resources.Project.build(Unknown Source)
    at com.ibm.designer.domino.ide.resources.project.BuildJob.runInWorkspace(Unknown Source)
    at org.eclipse.core.internal.resources.InternalWorkspaceJob.run(Unknown Source)
    at org.eclipse.core.internal.jobs.Worker.run(Unknown Source)