Same here, in-fact its not only different GET parameters but also for unique URL for different events for same app.
So in our case, during first registration we can add hooks on an app with endpoint "http://domain.com/hook/123" for CREATE and UPDATE events and next time when we try to add hooks with endpoint "http://domain.com/hook/456" for CREATE and UPDATE events we get this message "An existing active hook is already present for this client". Strange!!!
Something has really changed in the API layer that is causing this problem. Quick help on this is appreciated.