I've had the very same problem before, and the only solution that have worked for me have been to add a "when not exists" in the insert statement. Like this:
insert into table (key_id)
(select (:key_id_var)
where not exists
(select 1 from table where key_id = :key_id))
or just
select create__new(:key_id)
where not exists (select 1 from table where key_id = :key_id)
I have tried other approaches, like generating the object_id in another page (a confirmation one, for instance), but there are allways some cases when it fails (if something can go wrong will go wrong ;o)
Of course a better approach would be to somehow improve the already existing API of the ad_form or the acs_objects to avoid this problem...
