Currently running: 5.11.2-1-MANJARO (linux511)
The following kernels are installed in your system:
* linux419
* linux510
* linux511
* linux54
rename or remove in Directory /etc/mkinitcpio.d/
-rw-r--r-- 1 root root 392 Nov 2 16:12 linux57.preset
-rw-r--r-- 1 root root 392 Nov 1 10:51 linux59.preset
I removed the extra from /etc/mkinitcpio.d/ and then ran mkinitcpio -P and update-grub and both commands completed correctly.
and you are sure you are right on unstable branch ?!
Yes, I am on unstable branch.