Can I use identityHashCode to produce a compareTo between Objects respecting same-ness?

While it's not guaranteed, I suspect the chances of this causing a problem are vanishingly small.

System.identityHashCode returns the value that Object.hashCode would return if not overridden, including this in the documentation:

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects.

So is "as much as is reasonably practical" sufficient? While it's not guaranteed, I would be very surprised if you ever ran into a situation where it causes a problem. You'd have to have two messages with exactly the same timestamp and where the JVM's Object.hashCode implementation returns the same value for the two messages.

If the result of that coincidence were to be "nuclear power plant explodes" then I wouldn't risk it. If the result of that coincidence were to be "we fail to bill a customer" - or even "we bill a customer twice, and might get sued" I'd probably accept that chance, if no better alternatives are suggested.