nvidia-dkms-460 (and surely other versions) ask to enroll the ubuntu secure boot module signature key even if it was already enrolled.

[pae23]

I test on a fresh 21.04 (I enrolled the "ubuntu secure boot module signature key" to my MOK during the install process) and I check it was done after my first reboot (grep 509 in dmesg).
When I installed the nvidia-dkms-460, it ask me to add the key again, I followed the steps and it enrolled a second time this key (so now I have 2 time the same "ubuntu secure boot module signature key" with exactly the smae fingerprint in my MOK.

I think you can add a check in the pacakge to verifiy if the key is already enrolled and perhaps a check in the MOK manager to avoid enrolling 2 times the same key.