I have a Control inside a ControlNode with non-idempotent double-click behaviour (first time will work, second will cause an error). The problem was, the Diagram.NodeDoubleClick event was being fired twice each time. (Ditto for NodeClick where I was handling right-clicking, but that was less important.) I finally pinpointed the problem: the ControlNode is listening to the Control.MouseUp and Control.MouseDown events twice each. Our company having access to the source code, I identified the reason why. The problem appears to be that the ControlNode attaches to the Control events in both OnAdd and SetControl. That is, when the control is set, it registers the mouse event handlers, and then when the ControlNode is added to the Diagram, it registers it a second time.

I believe that the OnAdd/OnRemove ones should be removed. (Having fixed that, it works correctly and as I would expect it to.)

What our developer did is removing the handlers before adding new ones, both in OnAdd and SetControl methods:

// Detach any event handlers if they are already attached,
// to make sure we won't get double notifications
control.MouseDown -= OnControlMouseDown;
control.MouseUp -= OnControlMouseUp;

// Listen to mouse clicks over the control
control.MouseDown += OnControlMouseDown;
control.MouseUp += OnControlMouseUp;
 

The -= operator will do nothing if there aren't handlers attached, otherwise will remove current handlers. Attaching and detaching in OnAdd / OnRemove is necessary in some scenarios, e.g. if you move a ControlNode between different diagrams.

I hope that helps,
Stoyan