"Something as simple as a event id could have a rather complex semantics and you should be aware of this when designing your event model. Or is it just me complicating things?"
I agree that event semantics may be complex. If you have a three-car pile-up, does that count as one collision or two, given that the third car hits a few seconds after the first two? Or three collisions, if the third car hits both of the first two cars?
So you have to have some semantic model that provides a basis for counting how many collisions there are in the real world, let alone how many event reports are received. There is a semantic rule (sometimes called an identity rule, after Frege) as to when two things are the same, or the-same-again.
An identity rule (together with a membership rule - - which things count as collisions at all) should be part of your definition of “collision”.
See my earlier posts: How many products? and The Semantics of Security