The issue with not having a single source of truth is not the fact that you have to update code in 2-3 places, it’s that you have to know to update code in 2-3 places.

Accidental divergence is the problem, not intentional.

Yes, this is true. And is a bigger problem on large teams. One mitigation is a comment by the original author at both sites that there may be a coupling in the future.

But, again, the point is that you don't know yet whether you have a single source of truth or not. It's a question of the relative badness of duplication vs premature abstraction in cases where the code may diverge or converge in the future. There is no generic answer. But as a heuristic, based on my personal experience, I have always found premature abstractions to be more painful to work with. Even more so when someone else has authored them.