6 files declare their use of PreExistingConfigException in Drupal 9

ConfigInstallTest.php in core/tests/Drupal/KernelTests/Core/Config/ConfigInstallTest.php

ConfigInstallWebTest.php in core/modules/config/tests/src/Functional/ConfigInstallWebTest.php

ModulesListConfirmForm.php in core/modules/system/src/Form/ModulesListConfirmForm.php

ModulesListForm.php in core/modules/system/src/Form/ModulesListForm.php

ThemeController.php in core/modules/system/src/Controller/ThemeController.php

ThemeExperimentalConfirmForm.php in core/modules/system/src/Form/ThemeExperimentalConfirmForm.php