We duplicate code to avoid bugs. We have a lot of customer integrations, and often the source system is one we've integrated with before. So we usually copy th

We duplicate code to avoid bugs. We have a lot of customer integrations, and often the source system is one we've integrated with before. So we usually copy the whole integration and make some tweaks. Invariably our customers want some data massaging and custom logic, and by duplicating we can't screw up another customer's integration when we make changes. Of course we do have some code in shared libraries, like parsing dates and numbers, cleaning mixed utf-8/latin1 input and such. Of course, that does mean we might have to change many integrations if something fundamental changes. But overall it has worked very well for us.