This has a tendency to happen commonly with bots / crawlers / other automation tools but of course can happen naturally - as there’s nothing to guarantee that an event_id is truly unique at event time (this can be done but would be quite an expensive operation).
What QA tool are you using out of interest? Many crawlers / tools tend to avoid expensive operations (like generating random numbers as in the case of event_id, or by seeding the RNG ahead of time) so it’s not uncommon to see event_ids repeated.