I tend to follow the "rule of 3": a second similar implementation is OK, introducing the third triggers a refactor. As with everything, this isn't dogma, and sometimes the second implementation is already too much, while at other times you get tens of similar code sections (in codegen, repeating patterns with almost no changes is a virtue). But it's a good rule of thumb.

On testability: two implementations can be tested against each other, leading to greater coverage with less test code. It doesn't work that way for 3+ implementations, which is another reason not to have that many.