This is blocking the re-enablement of the "assert no runtime errors" step in CI, which checks that mw-error.log remains empty. This was accidentally removed a few months ago (T246358).
mw-dberror.log
Fri Mar 20 18:11:23 UTC 2020 32a44f17c9c0 wikidb-unittest_ User::updateActorId localhost:/workspace/db/quibble-mysql-fyqd_1b0/socket 1062 Duplicate entry 'TestNewSystemUser 81e504.1010' for key 'actor_name' (localhost:/workspace/db/quibble-mysql-fyqd_1b0/socket) INSERT INTO `unittest_actor` (actor_user,actor_name) VALUES (169,'TestNewSystemUser 81e504.1010')
#0 /workspace/src/includes/libs/rdbms/database/Database.php(1580): Wikimedia\Rdbms\Database->getQueryExceptionAndLog(string, integer, string, string)
#1 /workspace/src/includes/libs/rdbms/database/Database.php(1159): Wikimedia\Rdbms\Database->reportQueryError(string, integer, string, string, boolean)
#2 /workspace/src/includes/libs/rdbms/database/Database.php(2242): Wikimedia\Rdbms\Database->query(string, string)
#3 /workspace/src/includes/libs/rdbms/database/Database.php(2223): Wikimedia\Rdbms\Database->doInsert(string, array, string)
#4 /workspace/src/includes/user/User.php(4259): Wikimedia\Rdbms\Database->insert(string, array, string)
#5 /workspace/src/includes/user/User.php(4149): User->updateActorId(Wikimedia\Rdbms\DatabaseMysqli)
#6 /workspace/src/includes/libs/rdbms/database/Database.php(4179): User::{closure}(Wikimedia\Rdbms\DatabaseMysqli, string)
#7 /workspace/src/includes/libs/rdbms/database/DBConnRef.php(68): Wikimedia\Rdbms\Database->doAtomicSection(string, Closure)
#8 /workspace/src/includes/libs/rdbms/database/DBConnRef.php(641): Wikimedia\Rdbms\DBConnRef->__call(string, array)
#9 /workspace/src/includes/user/User.php(4156): Wikimedia\Rdbms\DBConnRef->doAtomicSection(string, Closure)
#10 /workspace/src/includes/user/User.php(805): User::createNew(string, array)
#11 /workspace/src/tests/phpunit/includes/user/UserTest.php(2134): User::newSystemUser(string, array)
#12 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestCase.php(1408): UserTest->testNewSystemUser(string, array, array, string)
#13 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestCase.php(1028): PHPUnit\Framework\TestCase->runTest()
#14 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestResult.php(691): PHPUnit\Framework\TestCase->runBare()
#15 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestCase.php(756): PHPUnit\Framework\TestResult->run(UserTest)
#16 /workspace/src/tests/phpunit/MediaWikiIntegrationTestCase.php(418): PHPUnit\Framework\TestCase->run(PHPUnit\Framework\TestResult)
#17 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestSuite.php(597): MediaWikiIntegrationTestCase->run(PHPUnit\Framework\TestResult)
#18 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestSuite.php(597): PHPUnit\Framework\TestSuite->run(PHPUnit\Framework\TestResult)
#19 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestSuite.php(597): PHPUnit\Framework\TestSuite->run(PHPUnit\Framework\TestResult)
#20 /workspace/src/vendor/phpunit/phpunit/src/Framework/TestSuite.php(597): PHPUnit\Framework\TestSuite->run(PHPUnit\Framework\TestResult)
#21 /workspace/src/vendor/phpunit/phpunit/src/TextUI/TestRunner.php(621): PHPUnit\Framework\TestSuite->run(PHPUnit\Framework\TestResult)
#22 /workspace/src/vendor/phpunit/phpunit/src/TextUI/Command.php(200): PHPUnit\TextUI\TestRunner->doRun(PHPUnit\Framework\TestSuite, array, boolean)
#23 /workspace/src/tests/phpunit/phpunit.php(71): PHPUnit\TextUI\Command->run(array, boolean)
#24 /workspace/src/maintenance/doMaintenance.php(99): PHPUnitMaintClass->execute()
#25 /workspace/src/tests/phpunit/phpunit.php(130): require(string)
#26 {main}