When adding aliases through the admin interface, there is a validation check (function aliasExists) to make sure you are not reusing an existing alias. Strangely the function does not prevent you from making a duplicate alias.
To reproduce, go to /admin/config/search/path/add and add an alias with a source. Then do it again (same alias, same source).
If you use different sources you will be stopped from reusing an alias, but nothing stops you from adding a duplicate. Why allow that?
