So you can't have exactly once semantics. When you say 'bus' I assume you mean a message broker which at best gives you at least once semantics. What happens when the message is delivered multiple times?
That's just a broker; all message handlers must be idempotent. This sometimes means storing message IDs or creating one-direction state change conduits that fail subsequent duplicate messages.
To answer that question more succinctly, nothing special happens when a message is delivered more than once.
I would assume you propagate a transaction ID or expected row version (and persist transaction histories and/or version numbers along with the inventory item row after mutation).
