Hi nesim yigit,
The events that you mentioned are fired whenever a group changes its state. Since one group becomes expanded and another group becomes collapsed, one event is fired two times - once for each group.
If you want only one global event which is fired only once during the expand/collapse operation, you need to subscribe to the events of RadPanelBar, for example PanelBarGroupExpanded:
sender, Telerik.WinControls.UI.PanelBarGroupEventArgs args)
I hope this helps. If you have additional questions, feel free to contact me.
the Telerik team