Typically, in the teams I've been on that did the best job of keeping the code clean, we'd handle this by just being good at code review: The person submitting

Typically, in the teams I've been on that did the best job of keeping the code clean, we'd handle this by just being good at code review: The person submitting the change might not remember any duplicates, but there's a good chance that one of the reviewers will, if that's one of the things they're watching for. Alternatively, if you really want to be exacting about this, there are code analysis tools that will do it for you.