Describe the bug
From :help :argedit:

When {name} already exists in the argument list, this
entry is edited.

This is not correct. If the file is already in the argument list, a new entry will be added to the list instead of editing that entry. This is either a bug or the documentation is incorrect. Personally I would prefer if the behavior was changed to match the documentation.

To Reproduce

1. vim --clean
2. :args 1.txt 2.txt 3.txt
3. :argedit 3.txt

Expected behavior
After doing the above steps, I expected:args to show

1.txt 2.txt [3.txt]

Instead, it shows

1.txt [3.txt] 2.txt 3.txt

Environment:

• Vim 8.2.592
• Windows 10
• mintty 3.1.7