So each time a post "just asking questions" is posted, close it with a link to the explanation of why the idea has already been debunked. Just like a Bugzilla dupe or a StackOverflow dupe.

For a long time, Reddit moderators did not have the tools to do this. Regardless, whenever bad-faith argumentation is removed, the person whose comment was removed paints the event as censorship which just backs up their conspiracy argument more

Now do this 1,000 times. Now 10,000 times. It is trivial to suck all the air out of a forum by repeatedly posting this.

Spam detection has been a feature of email clients for a long time. I'm sure even a rudimentary automated detection mechanism could prevent a significant portion of the dupes.

Is that not censorship then?

In order to have a good community, you need a form of censorship, whether through moderation, spam detection, voting systems, and so on. Otherwise, the community as a whole becomes a mess.

Somebody "just asking a question" is not being censored. Somebody "just asking a question" will be happy for the quick response.