For the “duplication” problem, I don’t know.
The reference for you to “study” is:
https://wiki.archlinux.org/title/GRUB
As you’ll see and probably know, /boot/grub/grub.cfg is generated automatically by a variety of options in /etc/default/grub and scripts in /etc/grub.d/.
You could easily solve the problem correcting /boot/grub/grub.cfg, but problably the issue will re-appear next time you update the grub.
Checking options in /etc/default/grub and scripts in /etc/grub.d/ could be quite complex, but you can try.
Maybe (but I’m really guessing here) you could chroot from an live USB (to start from a clean status, I don’t know), reinstall the bootloader and upgrade grub according to:
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