Mhwd-kernel -li shows one kernel,5.10 but on grub it shows 3 5.10 with 5.8 and 5.7

[freggel.doe]

There are remnants of old kernels in /boot - remove them manually:

$ sudo rm /boot/vmlinuz-5.[78]-x86_64 /boot/initramfs-5.[78]-x86_64.img
$ sudo update-grub