Architecture & practice·Complexity and how it accumulates
two functions looked identical, you merged them, and a month later you added a boolean flag to tell them apart.
Incidental duplication
Code that is the same today by coincidence rather than because it expresses one rule. Merging it couples two things that were free to change independently, and the coupling is usually discovered when one of them needs to change and the other must not. The flag parameter added to a shared function is the classic tell, and the fix is to split it back apart.