[feoren]

Absolutely right. One of the most important questions to ask an abstraction is: what can I not do with this? If the answer is "nothing -- you can do everything you could before", then the abstraction is an inner platform. The entire power that abstraction brings is in "focusing" on the problems we care about solving; it must make other problems impossible (ideally ones we don't care about). It follows from the No Free Lunch theorem.

One way to make sure your abstractions are focused on solving the right problems is to always define them based on what you need, not based on what you have. The root of the abstraction vs. duplication debate comes down to this. Indeed it's unhelpful to look at two pieces of code and say "these look the same; I will abstract them!". Instead you say "wow these have really similar needs; I will define exactly what that need is and they'll both ask for it."