you have to check via programming first.
a good way to avoid "checking" is sort the friends so that the lowest ID is always first.
you will need to clean up your current table to make this work.
first, delete the duplicates:
then update the remaining rows where the friends are not sorted properly:
from friends f1
join friends f2
on (f1.friend_a_id, f1.friend_b_id) = (f2.friend_b_id, f2.friend_a_id)
where f1.friend_b_id < f1.friend_a_id
, friend_a_id = friend_b_id
, friend_b_id = @temp
where friend_a_id > friend_b_id