We've got a large number of customer-specific file integrations, and a lot of them are indeed very similar as the customers have the same system on the other side. However almost all the time there's some tweaking needed. Customer A used field X for this but customer B used the field for that.

So if a new customer comes and need an integration to a system we already support, even if we think they'll start out being identical, we just copy the code.

Thing is, these things evolve. Suddenly we have to patch over some process-related issues in the other system for customer A, while customer B does not have that issue. Now we can fix A's integration without worrying at all about affecting B.

Of course we write library and helper functions, and use those actively throughout, so we only repeat the "top level" stuff.