In that situation, the correct thing to do is, when the two pieces drift away from each other, to recognize that they are no longer the same abstraction and to

In that situation, the correct thing to do is, when the two pieces drift away from each other, to recognize that they are no longer the same abstraction and to break the connection. That may be painful - you have to look at everywhere that abstraction is used and figure out which thing it really is, and change the code to reflect it. But if that's going to happen, then in the early days, a little duplication was probably better.