select * from
((select f1.x,f1.y from functions f1
inner join functions f2
on f1.x = f2.y and f1.y = f2.x and f1.x!=f2.x and f1.y!=f2.y)

union

(select x,y from
(select x,y,count(x),count(y) from functions
group by x,y
having count(x)>1)x))pairs
where x<=y
order by x;