You can get many of the benefits of having one country through mechanisms like free trade agreements, open borders, shared currency zones etc.
This is key in my opinion.
Duplicates - digital copies as opposed to genetic clones - might not require new training (unless a whole/partial restart/retraining was being done).
Wouldn't new training be strongly adaptive -- if not strictly required -- if the duplicate's environment is substantively different from the environment of its parent?
When combined with self-modification, there could be 'evolution' without 'deaths' of 'individuals' - just continual ship of Theseus processes. (Perhaps stuff like merging as well, which is more complicated.)
I understand this model; at the same time, however, it's my impression that it's commonplace in software development to periodically altogether jettison an old legacy software system in favor of building a new system from the ground-up. This seemed to be evidence that there are growing costs to continual self-modification in software systems that might limit this strategy.
I'll check it out -- thanks Zachary!