There are basically two patterns in avoiding duplicated registering of event handlers:
(According to this discussion: C# pattern to prevent an event handler hooked twice)
MyEvent -= MyEventHandlerMethod;
MyEvent += MyEventHandlerMethod;
According the documentation, invocation list is being stored as array or something similar to it, and the order of the event handlers is being stored too. May be there are inner structure to maintain fast search for a particular method there.
So in the worst case operation of the
O(1) (as we simply got the reference for the array) +
O(n) for searching the method, even if there is no optimization for it. I seriously doubt that this is true, and I think that there is some optimizing code, and it is
Second approach has additional operation of adding which is, I think,
O(1), as we adding the event handler to the end.
So, to see the difference between such actions, you need a lot of the event handlers.
But! If you use the second approach, as I said, you'll add the event handler to the end of the queue, which can be wrong in some cases. So use the first one, and has no doubt in it.