The question is whether updates can be made against both copies of the duplicate, requiring synchronization. It's the difference between a replicated slave copy of a database and a real distributed one. The first is easy. The second is very hard.
I'd argue if you find yourself needing to do this, it actually might be a hint that abstracting is appropriate, as it's proof there is more than superficial commonality. Like so many things, I don't think there's a way to make any hard and fast rules, and figuring this out is more art/experience/taste than science.
Not including cross-cutting concerns that modify all usages (eg, changing your logging or dependency injection library).
