After one of the recent update, my grub entries got multiplied. I thought this will go away with grub-update or some new, bigger update with kernels but no, it’s still there and it looks it won’t fix itself. I don’t have grub-customizer installed, so that’s not it.
Any idea how to fix it now? I know I could remove it manually from some file, but there must be some issue that caused it and would prefer to adress it, so the usual grub updates would restore entries correctly.
Put
GRUB_DISABLE_BOOTNEXT=true
to the end of your /etc/default/grub file and run
$ sudo update-grub
Then reboot.
In case you are surprised by many new entries in the GRUB menu, possibly also duplicates, this comes from a change in GRUB 2.16 which now reads the EFI variables too and processes them. To disable this new feature, in sudo nano /etc/default/grub after GRUB_DISABLE_OS_PROBER=false add
GRUB_DISABLE_UEFI_FIRMWARE=true
GRUB_DISABLE_BOOTNEXT=true
Save with ctrl+s, quit with ctrl+q, and update the grub menu with sudo update-grub.
Note that as of middle of October, this new features are still work in progress and behavior can and will change. Only try this if you actually have too many entries after the update.
I think this is not needed.
The situation is actually kinda unclear. In theory you are right - the one hides the firmware setup itself, the other the pulling of efi boot entries and converting them to grub entries. In practice last time i checked the source code at gitlab some days ago the toggle BOOTNEXT was not implemented at all… no idea which version is it.
So the best will be if the OP tests first with BOOTNEXT only and only if it does not work add the FIRMWARE. And report the results.
I added those, updated grub, restarted and it fixed my menu. I have 5 entries now instead like 20 or so… 
EDIT: I disabled GRUB_DISABLE_UEFI_FIRMWARE=true, rebooted and still have 5 entries, so I guess this wasn’t the issue here and disabling BOOTNEXT is enough.