Ref T13493. This check was introduced in D4647, but the condition can never be reached in modern Phabricator because the table has a unique key on <accountType, accountDomain, accountID> -- so no row can ever exist with the same value for that tuple but a different ID.

(I'm not entirely sure if it was reachable in D4647 either.)

Used SHOW CREATE TABLE to look at keys on the table and reasoned that this block can never have any effect.