Data point --- if you're a twin, and therefore have the same last name, and same birthday as someone ... United Airlines "apply travel credits" system explodes, and you have to call them.

A lazy Developer tried to supplant Last Name + Birthday as a unique identifier ... because ... what are the odds, right? .... And we know Marta Manuel Cortez De Los Lobos Hernandez has broken their system too may times, hence ... Last Name + Birthday.

Social security number would be better; but that might depress conversion rate. People are reluctant to enter that even though you can purchase anyone's SSN for $5 online from various nefarious websites.

Anyway I called them & they worked it out. Shame, it almost worked in my favor with "fungible" credits.