--- modules/path/path.module
+++ modules/path/path.module
@@ -97,10 +97,11 @@
     // Check for existing aliases.
     if ($alias == drupal_get_path_alias($path, $language)) {
       // There is already such an alias, neutral or in this language.
-      // Update the alias based on alias; setting the language if not yet done.
-      db_query("UPDATE {url_alias} SET src = '%s', dst = '%s', language = '%s' WHERE dst = '%s'", $path, $alias, $language, $alias);
+      db_query("UPDATE {url_alias} SET src = '%s' WHERE dst = '%s' AND language = '%s'", $path, $alias, $language);
     }
     else {
+      // Ensure that no similar aliases exist.
+      db_query("DELETE FROM {url_alias} WHERE dst = '%s' AND language = '%s'", $alias, $language);
       // A new alias. Add it to the database.
       db_query("INSERT INTO {url_alias} (src, dst, language) VALUES ('%s', '%s', '%s')", $path, $alias, $language);
     }