The PHP array union operator += does not work as intended in [[https://phabricator.wikimedia.org/diffusion/EARU/browse/master/src/UserNameSessionProvider.php|src/UserNameSessionProvider]] on lines 459 and 571, because it it used on non associative arrays.
Both arrays will be changed to associative arrays first before the operator is getting applied. But then the compared keys are integer indices, and they exist in both arrays.
Instead of the union operator += the functions array_merge and array_unique have to be used on non associative arrays.