code: public FaunusPipeline dedup() {
          this.state.assertNotLocked();
          this.state.assertNoProperty();
  
          this.compiler.addMap(DuplicateFilterMap.Map.class,
                  NullWritable.class,
                  FaunusVertex.class,
                  DuplicateFilterMap.createConfiguration(this.state.getElementType()));
          makeMapReduceString(DuplicateFilterMap.class);
          return this;
      }
docs: Remove any duplicate traversers at a single element.
  
  @return the extended FaunusPipeline