I've seen lots of code that people have maintained for 20 years and its full of these duplication and worse. In fact I'm sad to say that majority of code I've seen people write and maintain is worse than what LLMs produce today. Often it is inexperience, sometimes it is willful negligence, but most often it is just tight deadlines and pressure to do finish whatever is being done right now. People know how to do it better, but nobody got the time and budget to actually do it. LLMs also learned from that.