hello,
after saying four times that I finally understand event-bindings,
I'll quit forever 
Here is some code I really don't understand,
on the other hand,
now writing this in shorthand, I think I see the trick.
The problem is that in the code below,
the OnEndDrag handler of somehigherbase is called twice.
So each bind command apparently adds a pointer to the list, whether it's already there or not.
So in this case I need to solve that by giving the second "OnEndDrag" another name,
so maybe I should another golden rule: "never use the name of the event handler of the derived class".
Maybe this is useful information for others ( at least it was for me
cheers,
Stef
class somebase (CT.CustomTreeCtrl) :
  def __init__ ():
    ....
    self.Bind ( CT.EVT_TREE_END_DRAG, self.OnEndDrag )
  def OnEndDrag ():
    ... do something,
    but no Skip()
class somehigherbase (somebase):
  def __init__ ():
    ....
    self.Bind ( CT.EVT_TREE_END_DRAG, self.OnEndDrag )
  def OnEndDrag ():
    ... do something
    event.Skip()