Under the assumption that the same event fired from the same IP at the same time, with the same environment, etc. would be considered a duplicate in your system

Under the assumption that the same event fired from the same IP at the same time, with the same environment, etc. would be considered a duplicate in your system, I'd have designed this system to be idempotent-insert-only and use content-addressing instead of nonces for identity (event ID = suitably large hash of event data to avoid collision). If that assumption doesn't hold, then add your nonce to the event data (and thereby modify the hash).