A screenshot of what you’re seeing would have been helpful. In the absence of that, I’ll provide my own screenshot of what I think you’re seeing:
This duplication issue is raised at file level to tell you that somewhere in the file are some number (2 in my example) of duplicated blocks. If you scroll down through the file, you’ll see marginal markers that indicate which blocks are duplicated. E.G.:
The interesting thing here is that you’re apparently seeing a newly-raised duplication issue after changing only one line. The docs outline how duplication detection works. At a guess, I’d say your one line, pushed some block of code over the threshold.
Of course, this is all guess-work based on an intuitive reading of your report. Feel free to come back and tell me I’m wrong.