It has happened several times that users wanted to log into Phabricator via the SUL/OAuth/mediawiki.org account, got to a registration screen, and then got an error in Phabricator that their email address is already taken.
Usually this happens when the user already registered in Phabricator once upon a time via their LDAP/wikitech/Gerrit/Developer account.
After successfully logging in, users can add their SUL/OAuth credentials at https://phabricator.wikimedia.org/settings/user/YOURUSERNAME/page/external/
We should document this on https://www.mediawiki.org/wiki/Phabricator/Help
Brought up by @Lex at Wikimedia-Hackathon-2018