It’s been a while since I used Aldon, and I may be getting it confused with Turnover, but I seem to recall that Aldon/CMS warned you that an object was already checked out and gave you the option to proceed and check it out anyway.
The first person to check it back in had their changes accepted and the second person was advised by the system that the base version had changed since check out and to review for changes.
If I remember correctly there was also an option to perform emergency check outs where, for example, a program failed in production that was already checked out for development. You could do an emergency check out, mod and check in and the developer who had the program checked out originally would be advised to review the changes made.
Hope it helps