On a file the a copyright header:
contrib/devtools/copyright_headers.py insert <file>
This should outout the following error and make no change:
*** src/seeder/main.cpp already has a copyright by The Bitcoin developers
Remove the `// Copyright (c) <oldyear>-<newyear> The Bitcoin developers` line from a `.cpp` or `.h` file.
contrib/devtools/copyright_headers.py insert <file>
git restore file.cpp
The copyright header should be the same as before (maybe newyear is updated to something more recent).
Edit the `// Copyright (c) <oldyear>-<newyear> The Bitcoin developers` line in a `.cpp` or `.h` file to `// Copyright (c) <oldyear>-<newyear> The Bitcoin Core developers`.
contrib/devtools/copyright_headers.py insert <file>
The copyright header should now include the `The Bitcoin developers` line (which could have its newyear updated as before) underneath the `The Bitcoin Core developers` line.
Completely remove the entire copyright header from the file.
contrib/devtools/copyright_headers.py insert <file>
contrib/devtools/copyright_headers.py insert <file>
This should reproduce the results of the second case.
Repeat using a python file.