What's funny is that DRY was first popularised in the Pragmatic Programmer[0] book, and "coincidental" duplication is explicitly addressed right there on page 34, "not all code duplication is knowledge duplication... the code is the same but the knowledge is different... that's a coincidence, not a duplication."

[0] https://www.amazon.co.uk/Pragmatic-Programmer-journey-master...

Fortunately, there is a newer edition ;) The Pragmatic Programmer, 20th Anniversary Edition: https://www.amazon.com/Pragmatic-Programmer-journey-mastery-...

+1 on CI & automated tests BTW.

A new version of "The Pragmatic Programmer" recently came out. [EDIT: not available yet, only preorder at amazon, beta version available at pragprog.com.] That book is all about tools and methods that a self-taught programmer should look into:

https://www.amazon.com/Pragmatic-Programmer-journey-mastery-...