Event handlers are not uniquely identified

[dktapps]

Description

Event handler functions need to have a deterministic identifier to prevent duplicate registrations.

Justification

It is currently (theoretically) possible to register the same handler function to listen to an event multiple times, usually by mistake. When this happens, it works perfectly fine, until your handler function is called multiple times when the event fires.

We need a deterministic way to identify handler functions to prevent them from being registered multiple times.

[SOF3]

I encountered this issue when implementing #2470 too. I was trying to identify the current context that is trying to continue the event, but that requires a bit of hacking, mainly because of the unreliability of the manner that the event call happened.