@Override
    public Set<String> getScopeKeys() {
        return null == this.dedupLabels ? Collections.emptySet() : this.dedupLabels;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/filter/DedupGlobalStep.java