onBoxSelect handler will no longer allow nodes to be selected more than once.

src/editor/editor.tsx

@@ -188,7 +188,9 @@ export class Editor extends React.PureComponent<any, stateTypes> {
             return;
         }
 
-        this.selectNodes(selectedNodes.concat(this.state.selectedNodes));
+        this.selectNodes([
+            ...new Set(selectedNodes.concat(this.state.selectedNodes)),
+        ]);
     }
 
     /**