In an Ubuntu terminal window, type:
sudo efibootmgr -v
This will show you details for all the boot entries in your EFI's boot manager. Examine the details for the two
ubuntu entries to see if they differ. If they don't, it's safe to remove one of them via the
-b # -B option, as in:
sudo efibootmgr -b 4 -B
This example deletes the
Boot0004 entry. I recommend deleting the entry that comes later (or not at all) in the
BootOrder list (which is also shown by
It's also safe to delete an entry if it refers to a non-existent file, partition, or disk. Unfortunately, the partition and disk identification is done via long strings that are difficult for humans to parse. You might have such an old entry if you installed Ubuntu twice.
Note that there are certain bugs that can cause these entries to multiply. In particular, an interaction of a buggy EFI and a buggy version of Shim can cause this problem. If you run into this condition, I recommend disabling Secure Boot and bypassing Shim. The former requires entering your firmware setup utility, finding the Secure Boot option (its location and name varies, unfortunately), and disabling it. The latter requires creating a new NVRAM entry that points directly at GRUB, rather than at shim:
sudo efibootmgr -c -l \\EFI\\ubuntu\\grubx64.efi -L "GRUB direct"
IIRC, the buggy version of shim is distributed with Fedora, and Ubuntu's version is unaffected, so chances are you should not see this problem if you're using Ubuntu's Shim.