[crazy5sheep]

I don't agree the issues OP later discovered has anything related to `refactoring` itself, but more a issue of premature optimization.

my 2 cents, an interface is defined, it shall not be modified for no good reason. Even if you do, you can still have some way to make sure it can be compatible with the original system. and I don't believe you can't extract some common behaviors of those repeated code, and use them within the interface, refactoring doesn't mean you have to rewrite the whole project, it can be done by piece by piece. reducing a line of duplicated code can save you a lot of efforts on maintaining the project in its life-cycle. a lot of times, I have seen a code change was made to fix some bugs were forgotten in other place which duplicated the same original code.