diff --git a/plugins/org.eclipse.qvtd.compiler/src/org/eclipse/qvtd/compiler/internal/qvtp2qvts/AbstractSimpleNode.java b/plugins/org.eclipse.qvtd.compiler/src/org/eclipse/qvtd/compiler/internal/qvtp2qvts/AbstractSimpleNode.java
index 9c3eff3..4b4b827 100644
--- a/plugins/org.eclipse.qvtd.compiler/src/org/eclipse/qvtd/compiler/internal/qvtp2qvts/AbstractSimpleNode.java
+++ b/plugins/org.eclipse.qvtd.compiler/src/org/eclipse/qvtd/compiler/internal/qvtp2qvts/AbstractSimpleNode.java
@@ -31,8 +31,9 @@
 
 	@Override
 	public void addTypedElement(@NonNull TypedElement typedElement) {
-		boolean wasAdded = typedElements.add(typedElement);
-		assert wasAdded;
+		if (!typedElements.contains(typedElement)) {
+			typedElements.add(typedElement);
+		}
 	}
 
 	@Override