Don't you think it would be a nice feature to show previous similar posts when a user is about to submit an Ask HN post? I think Stackoverflow does smething like that.

I think that would be really cool. However, my understanding (completely from the outside) is that this site is deliberately minimal. I have no inside knowledge, but it seems that the number of features is as small as can be had without making the site effectively unusable.

You could set up another site to accept submissions, and have that site do the checks, return sensible previous submissions if found, and it not found, then push the submission through to here. Sub-optimal, but a cool hack.

I don't have the abilities for that. Maybe someone else has.