You prefer to block any "non-canonical" URL

No, just the ones without ".html" suffix which are obviously garbage and never existed on my website.

Do you mean the canonical tag solution is more safe, because it doesn't give user a bad experience if the macro doesn't give the correct URI in special cases? If so, ok, I see. But if not, I would ask: If you are able to determine the correct URI, why not making a 301 redirect? In my understanding the use of canonical tags is more proper on duplicate content pages that you WANT to have, i.e. page versions for different devices, print layout pages and such stuff.

the problem is to write a rule that matches only the "wrong URLs" to redirect to. I can't figure it out, cause you can build as many wrong URL as you want

Turn it around: You don't need to figure out all the wrong URLs, but all other than the correct one to the same content ID. It doesn't matter if the wrong URL is "/cat/12-blabla/" or "/catttt/12-blublue.html" - it is not correct "/category/12-page.html" for ID 12.

I assume that we can rely on the content IDs of Joomla. They are unique. And you know the correct URLs: category + alias + html suffix. --> And then all URLs which contain the same ID, but have a different URL are garbage. Isn't that the whole trick?

I can imagine that there are some case where this wouldn't work. But this "AUTOMATIC REDIRECT" could be switched off in the backend. Wouldn't that be genius: an Automatic Redirect! ReDJ would always deliver the correct URL, no matter how many and from where to where you move and rearrange your content, as long as the ID is the same. If the ID is changed, well, then you still need to add a redirect rule. But those rules could now simple be: redirect from ID 12 to ID 15. ReDJ knows the correct URL (as Joomla knows it, but doesn't proofe it) and redirects all URLs with ID 12 and all garbage URLs with ID 15 to the one correct URL with ID 15.

For some special cases, there could also be exclusion rules in ReDJ, which say: "don't touch URL /cat/13-example.html" or "don't touch all URLs with ID 13" at all.

In my opinion, such an Automatic Redirect killer feature would easyly justify a kind of "Enterprise +++" version of ReDJ for which I would have payed 90 Euro instead of 9. it would have saved me the time for more than 200 manual redirect rules. And it would perhaps als have an even better system performance.

That are great news! You are going to offer an extension that fights a loooong known, important mis-behaviour of Joomla! With the same quality as ReDJ and TagMeta, this will become a really famous extension, I bet.

I don't know who is your informer...

No informer. This problem is as old as Joomla itself. And if one cares about duplicate content, he/she can sometimes have doubts, if Joomla is the right choice at all.

Do you have an approximate time frame when the release could happen? (in summer? this year? ...)


Cheers
Marcel