Proper customer relations would have the website be smart enough to say, "ahh you are already registered for these promotions that overlap with this one [say, by ticketing and travel periods]... please pick one." Then you dont have to deal with any downstream logic to that effect...

But thats just too complex for a multi-billion dollar company to develop... sigh...

By the way, that was a well-known "gotcha" with that promotion (here on FT), but I know we dont all have time to investigate each promotion thoroughly before we register...