add double linked refs only once

src/ua/UABaseNode.ts

@@ -37,9 +37,11 @@ export class UABaseNode implements IToXML{
                 case ReferencyType.HasProperty:
                 case ReferencyType.HasSubtype:
                     if(ref.isForward) {
-                        this.children.push(node);
+                        if(!this.children.includes(node))
+                            this.children.push(node);
                     } else {
-                        node.children.push(this);
+                        if(!node.children.includes(this))
+                            node.children.push(this);
                     }
                     break;
                 }