How to ensure an event is only subscribed to once

if (*not already subscribed*)
{
    member.Event += new MemeberClass.Delegate(handler);
}

private bool _eventHasSubscribers = false;
private EventHandler<MyDelegateType> _myEvent;

public event EventHandler<MyDelegateType> MyEvent
{
   add
   {
      if (_myEvent == null)
      {
         _myEvent += value;
      }
   }
   remove
   {
      _myEvent -= value;
   }
}

if (alreadySubscribedFlag)
{
    member.Event += new MemeberClass.Delegate(handler);
}

class MemberClass
{
        private EventHandler _event;

        public event EventHandler Event
        {
            add
            {
                if( /* handler not already added */ )
                {
                    _event+= value;
                }
            }
            remove
            {
                _event-= value;
            }
        }
}

myClass.MyEvent -= MyHandler;
myClass.MyEvent += MyHandler;