It may be trivial, but I think the answer is: they don’t double check. Possibly that’s too much effort to do so?

I might be wrong on how these spam bots operate, but I assume someone (human) has to write at least a few lines of scripts tailored to the form on the website, to actually submit the spam. Adding a few more lines to also check that the submission went through doesn't seem like much effort.

Some of the "spammers" are just mentally ill people. The unethical practice of shadowbanning is highly effective against them.

> I don't understand why shadowbanning would be so effective

Because if done correctly the user never knows they are shadow-banned. It sounds trivial when you know _how_ the shadowban is done. But for instance, instead of an IP check, perhaps it's a time check - after 3 days it comes into play. Or a combination of different checks. So imagine that you are accessing a service that appears to be working correctly .... you would basically need to a) determine that that service even does shadowbanning, and b) think of infinite ways that you might be shadowbanned and try to determine if that's the case.

> The risk of misidentifying legit users and shadowbanning them

What's the risk?

A legit user getting told they are banned can contact the site to try and resolve the issue on why they have been misidentified, getting shadowbanned will possibly never get resolved.

Not just never resolved but potentially waste large amount of their time making posts that will never be seen by anyone else.