Exactly this. I fixed a problem like this a week ago. I found some duplicated code, factored it out into one place by introducing an abstract base class (Python

Exactly this. I fixed a problem like this a week ago. I found some duplicated code, factored it out into one place by introducing an abstract base class (Python) and in the process discovered one of the duplicated methods had a logic error leading to returning a slightly smaller integer result. The code had test coverage, but the test confirmed that it produced the wrong result. I had to fix the test too.