[simplotek]

> Should both my service and Galactus have full, individual copies of this data structure?

Yes. I recommend you read "Domain-driven design" by Eric Evans, specially the part explaining the concept of a bounded context.

> Alternatively, if Galactus has a published schema that I refer to, isn't that a shared data structure?

No, that's the interface.

> not only we deduplicate code

You don't. You just needlessly add a constraint for no reason at all, and in the process make your life a mess.