The “don’t repeat yourself” principle is well established, but over-aggressive refactorizarions to extract common code are also widely known for creating hard to maintain code due to the introduction of tight coupling between components that should not be coupled. A passing resemblance between code blocks is reason enough to extract them away, even if that ends up breaking Liskov’s substitution principle.
To mitigate problems caused by DRY fundamentalisms, the “write everything twice” (WET) principle was coined. WET works by postponing aggressive refactorizarions, the kind that introduces complexity and couples unrelated code just because it bears some resemblance, by creating a rule of thumb where similar code blocks showing up twice in the code should not be refactored, and only code that shows up multiple times should be considered for this task. However, this rule ignores context and nuances, and can dissuade developers from cleaning up code.
So, where do you stand on the topic? How do you deal with duplicate code? Do you follow any specific rule of thumb?
It depends on how much regression testing is in place to test that old and refactored code behaviours have the same outputs, and how much budget there is for writing this tests.
For old financial systems for example, the answer is often to repeat the code again, as there’s likely to be little tests to confirm existing behaviour and writing tests around very complex business domains is prohibitively expensive.