We were able to reproduce the issue only while debugging.
The event is triggered only once when you change the value in the control via typing in the textbox or clicking the arrows. But when you place a breakpoint in the event it is hit multiple times. This is a common issue in some events which causes the breakpoints to be hit multiple times and at the same time behave as expected when not in debug mode. This issue may be caused by the focus getting reset to the control or another related scenario.
I would recommend to you to debug this and any other events in which you experience this issue with for example Debug.WriteLine() method without explicitly placing breakpoints in the event.
Hope this information is helpful.
the Telerik team