get duplicateChildren() {
      const existingChildren = this.targetElements.flatMap((e2) => [...e2.children]).filter((c2) => !!c2.id);
      const newChildrenIds = [...this.templateContent?.children || []].filter((c2) => !!c2.id).map((c2) => c2.id);
      return existingChildren.filter((c2) => newChildrenIds.includes(c2.id));
    }
Gets the list of duplicate children (i.e. those with the same ID)