The AppointmentResizeEnd event fires in cases where there was no resize done (for example when selecting appointments)

To workaround:

public Form1()
{
    InitializeComponent();
    this.radScheduler1.SchedulerElement.ResizeBehavior = new MyResizeBehavior(this.radScheduler1);
}
 
class MyResizeBehavior : AppointmentResizingBehavior
{
    public MyResizeBehavior(RadScheduler scheduler)
        :base(scheduler) { }
 
    public override bool EndResize(IEvent appointment)
    {
        if (!this.IsResizing)
        {
            return false;
        }
        return base.EndResize(appointment);
    }
}