Fix importScene with external component

When importScene is used directly with external component, don't add
content as children again.

Task-number: QTBUG-83141
Change-Id: Ibe92c9fede694bd2df5b474d2f0132f117eb5b97
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>
Reviewed-by: Antti Määttä <antti.maatta@qt.io>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

src/quick3d/qquick3dscenerenderer.cpp

@@ -341,8 +341,14 @@ void QQuick3DSceneRenderer::synchronize(QQuick3DViewport *item, const QSize &siz
         if (m_importRootNode)
             removeNodeFromLayer(m_importRootNode);
 
-        if (importRootNode)
-            m_layer->addChildrenToLayer(*importRootNode);
+        if (importRootNode) {
+            // if root node has already parent at this point
+            // it means "importScene: MyScene { }" type of inclusion.
+            // In this case don't duplicate content by adding it again.
+            const bool isEmbedded = importRootNode->parent;
+            if (!isEmbedded)
+                m_layer->addChildrenToLayer(*importRootNode);
+        }
 
         m_importRootNode = importRootNode;
     }