With patch set https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GlobalBlocking/+/971561 a phan stub for a class of the UserMerge extension was added (as that is a non-wmf deployed extension and it feels better to not have a dependency for it on CI and local developer).
But this new stub brings the CI of CheckUser into error:
src/HookHandler/UserMergeHandler.php:7 PhanRedefinedInheritedInterface \MediaWiki\CheckUser\HookHandler\UserMergeHandler inherits Interface \MediaWiki\Extension\UserMerge\Hooks\AccountFieldsHook declared at ../../extensions/GlobalBlocking/.phan/stubs/AccountFieldsHook.php:8 which is also declared at ../../extensions/UserMerge/includes/Hooks/AccountFieldsHook.php:12. This may lead to confusing errors.
CheckUser has UserMerge extension as phan dependency and loads it with:
$cfg['directory_list'] = array_merge( $cfg['directory_list'], [ '../../extensions/CentralAuth', '../../extensions/EventLogging', '../../extensions/GuidedTour', '../../extensions/GlobalBlocking', '../../extensions/UserMerge' ] );
That also pull in the .phan/stubs/ folder with the newly added stub and reports the duplicate interface error.
There should be a way to avoid that stubs are used by dependency or maybe only used when the real extension is not there.