If $wgCentralAuthAutoMigrate is set to true, when a user logs in and has no global account, in case all local accouns with same username match the email and/or password, create a global account and merge everything.
It was noted on https://meta.wikimedia.org/?oldid=9740729#RenameUser_announcement that this case is fairly frequent and not handled by $wgCentralAuthAutoMigrate yet. Bug 69291 should have taken care of all the unmerged accounts sharing email.
Example: https://tools.wmflabs.org/meta/stalktoy/Nemo+nonies , two local accounts with same password and no email set; I logged in for both and no global account was created.
Version: unspecified
Severity: enhancement