Duplicate key value violates unique constraint "path_alias____pkey"

If you get an error that looks like this while saving a node or a taxonomy term in Drupal, it may mean that the table that stores path aliases in the database has become corrupted for some reason.

Drupal\Core\Entity\EntityStorageException: SQLSTATE[23505]: Unique violation: 7 
ERROR: duplicate key value violates unique constraint "path_alias____pkey" DETAIL: 
Key (id)=(30) already exists.: INSERT INTO "path_alias" ("revision_id", "uuid", 
"langcode", "path", "alias", "status") VALUES (:db_insert_placeholder_0, 
:db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, 
:db_insert_placeholder_4, :db_insert_placeholder_5) RETURNING id; Array ( ) in 
Drupal\Core\Entity\Sql\SqlContentEntityStorage->save() (line 815 of 
/home/www-data/packages/drupal/web/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php).

Since aliases can always be regenerated (if you are using the Pathauto module), you can safely delete them. So, to fix the error truncate both path alias tables:

$tables = [
  'path_alias',
  'path_alias_revision',
];

foreach ($tables as $table) {
  \Drupal::database()->truncate($table)->execute();
}

then proceed to the Bulk generate page (admin/config/search/path/update_bulk) and generate path aliases again.

Before doing this make sure that you don't have any manually entered ('Generate automatic URL alias' unchecked) path aliases.

Warning: If you have manually entered path aliases then don't use this method to fix the error, because you will lose content that can't be regenerated automatically.