I will try to explain what happens.
Customers have requested that this event is fired both
when the item property definition changes and the filter operator changes. When the item property definition changes the event is fired for the first time. Since all filter operators are cleared and then re-populated for the new chosen item property, the selected operator also changes and fires the event for the second time. Since customers wanted this event to be fired on filter operator change so they can respond and modify the editor according to the chose operator.
So this is more like by design and not exactly a bug.
Is the fact that the event is fired twice somehow breaking your program logic severely. What are you doing in the event handler that should not be done twice?
All the best,
the Telerik team