The general approach is to refactor/generalize when creating a third version of something. With the first thing, you don't know if it needs any common functiona

The general approach is to refactor/generalize when creating a third version of something. With the first thing, you don't know if it needs any common functionality or what an appropriate abstraction will look like. With the second thing, you have some similarities but not enough information to know where the abstractions should be -- here, repeating yourself is OK. With the third thing, you should have enough information to work out where generalizations should be. Even then, only generalize what you need to at the time. Going overboard can add unnecessary complexity, so it is generally a good idea to be conservative in what you generalize. As you add more things, you can refine and evolve the system as needed. At this time you should have a better understanding of the system and what parts can be shared and generalized.