If you notice that two parts of the code look similar, but have a good reason not to merge or refactor, that deserves a signpost comment.
If you're copying and pasting something, there probably isn't a good reason for that. (The best common reason I can think of is "the language / framework demands so much boilerplate to reuse this little bit of code that it's a net loss" — which is still a bad feeling.)
If you rewrite something without noticing that you're doing so, something has definitely gone wrong.
If a client's requirements change to the point where you can't accommodate them in the nicely refactored function (or to the point where doing so would create an abomination) — then you can make the separate, similar looking version.
If you're copying and pasting something, there probably isn't a good reason for that. (The best common reason I can think of is "the language / framework demands so much boilerplate to reuse this little bit of code that it's a net loss" — which is still a bad feeling.)
If you rewrite something without noticing that you're doing so, something has definitely gone wrong.
If a client's requirements change to the point where you can't accommodate them in the nicely refactored function (or to the point where doing so would create an abomination) — then you can make the separate, similar looking version.