Avoid adding twice the same property (we should also have a test for that)

MaterialGraphProject/Assets/UnityShaderEditor/Runtime/Util/PropertyGenerator.cs

 
         public void AddShaderProperty(PropertyChunk chunk)
         {
+            if (m_Properties.Any(x => x.propertyName == chunk.propertyName))
+                return;
             m_Properties.Add(chunk);
         }