Erwin sent a correct reply. My next reply is related to possibility to disable overloading.
It is not possible to disable overloading – this is a base feature of PostgreSQL function API system – and cannot be disabled. We know so there are some side effects like strong function signature rigidity – but it is protection against some unpleasant side effects when function is used in Views, table definitions, .. So you cannot to disable it.
You can simply check if you have or have not overloaded functions:
postgres=# select count(*), proname from pg_proc where pronamespace <> 11 group by proname having count(*) > 1; count | proname -------+--------- (0 rows)