You hear it since you started programming: you have to remove, remove and remove code duplication!
Why? If you have ever worked with a legacy project, when there was code duplication, the same bug in the code had to be corrected in several places, which drove you crazy. And I don’t even talk about introducing new features.
Even quality tools like SonarQube tells you about code duplication percentage, with a heavy hint: if you have duplication, it’s bad. And if your manager sees these percentages, he may show up asking you “Why are we having 6% duplication on this project? You need to do something about it!”.
And in the end, they’re right: removing code duplication eliminates code redundancy in order to make your product easier to maintain and to add new features. It’s the famous principle of DRY: Don’t Repeat Yourself“.