Recently I setup an old PC to act as a test server to mimic a server I have for production. I decided I would like it to dual boot into Kali as well.
The installation went well however after the install finished on CentOS 7 would show in Grub. The Kali installation did not show any problems at all.
For anyone having similar issues after some research I was able to resolve the issue by completing the following command within CentOS:
grub2-mkconfig -o /boot/grub2/grub2.cfg
After running the command it should be clear immediately if this has worked as it will list the operating systems that it finds.
Hopefully this should save someone some time if they come across the same issue.