Code

private void calendar_ItemModifying(object sender, ItemModifyConfirmEventArgs e)
{
	var items = calendar.Schedule.GetAllItems(e.Item.StartTime, e.Item.EndTime.AddTicks(-1));
	if (items.Contains(e.Item))
		items.Remove(e.Item);

	e.Confirm = items.Count == 0;
}

private void calendar_ItemCreating(object sender, ItemConfirmEventArgs e)
{
	var items = calendar.Schedule.GetAllItems(e.Item.StartTime, e.Item.EndTime.AddTicks(-1));
	if (items.Contains(e.Item))
		items.Remove(e.Item);

	e.Confirm = items.Count == 0;
}