Pathauto does take some steps to prevent alias duplication in the PauthautoGenerator::createEntityAlias however if someone alters the path_alias in hook_path_alias_presave() it's possible they could still create a duplicate. One common example is to set the langcode for the default language to und, so that all translations can share the same alias. This can create a duplicate alias everytime someone saves a node in the default language if you are using the patch in
https://www.drupal.org/project/pathauto/issues/3113395
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Prevent redundant alias update caused by path alias language fallback.
Needs review
and setting the language to und, as the AliasStorageHelper::loadBySource method won't fall back to undefined anymore if the current entity langcode isn't found for that alias.
* Use the patch in
https://www.drupal.org/project/pathauto/issues/3113395
✨
Prevent redundant alias update caused by path alias language fallback.
Needs review
* Create a hook_path_alias_presave hook that sets any alias saved in the default language to und.
* Resave the node, it will always see the alias as new because you alter the langcode after the duplicate checks have been completed.
* Alternatively just create a duplicate programmatically.
Throw an EntityStorageException if a duplicate is found in a presave hook and log a warning.
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Needs review
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Code
Closing this as it can be prevented by altering the langcode in hook_pathauto_alias_alter rather than hook_path_alias_presave. Pre save occurs too late but the other one is fired before we save the alias so it prevents the duplicate issue.
Also, I guess that the core path_alias should be the one preventing duplicates being saved.