[thdc]

Knowing to DRY there depended on business knowledge that the original author did not have.

While they were wrong in this case, I would say it was a reasonable move to not DRY based on the code pattern itself at the time. And that's the big difference imo - DRYing based strictly on the structure of code vs business processes.

[chii]

But this implies that you have to guess when and where to DRY, which basically implies that there's no good way but via experience and domain knowledge!

That's not what people want to hear - they want a silver bullet; a set of criteria for which DRY'ing could be determined from the onset!