Binding a property multiple times should be an error

The compiler right now allows multiple bindings of the same property. E.g.:

Rectangle {
  width: 300
  height: 200
  color: "blue"
  color: "red"
}

This does not result in an error or warning - the last binding of color 'overrides' the previous one. This is IMO unexpected, and can easily lead to effects which are hard to understand.

Current Behaviors, though, rely on this feature, e.g.:

     
    y: 200  //initial value
    y: Behavior {
         NumberAnimation {
             easing: "easeOutBounce(amplitude:100)"
             duration: 200
         }
     }

This could maybe be also be tackled by having a 'initialValue' property in the Behavior definition.