Is this really really as designed or defined in some standard

It seems that we allow several function arguments to have same 
name (or is it label :)

hannu=# create or replace function ff(a int, a int) returns int language
plpgsql as $$begin return $1+$2; end;$$;
CREATE FUNCTION
hannu=# select ff(1,1);ff 
---- 2
(1 row)

hannu=# select ff(1,2);ff 
---- 3
(1 row)

hannu=# create or replace function ffa(a int, a int) returns int
language plpgsql as $$begin return a + a; end;$$;
CREATE FUNCTION
hannu=# select ffa(1,2);ffa 
-----  2
(1 row)

Is this defined by some standard or just an oversight ?

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Hannu