It would certainly depend to some degree on the complexity of the assignment. But it's also not that unusual for legitimate, non-plagiarized submissions to have dead code.
Sure, but is it not unusual to have "dead code consisting of lines from the original file repeated in random locations"? That would certainly stick out in any other environment (like a professional one).
I didn't study anything related to computers/software/programming in school, so I don't know what level is expected. But if I was tutoring someone and they handed me something with dead code in random locations in it, it would certainly catch my attention.
1. Students will frequently just try things until it works, move code around, etc., leading to very messy code.
2. Graders often do not look at individual assignments unless there is a reason to do so, often relying on automated test suites. And when they do look, I'd bet their first reaction is something like "I don't know why they're repeating themselves like this, but my rubric only penalizes them for 5 points here..."
