Help please,

The K2 Blog generates lots and lots of duplicate addresses, for example:

www.haprofessor.com/blog/item/51-ובהתייחס-ללמידה-פעילה.html

www.haprofessor.com/blog/item/51-12345

www.haprofessor.com/blog/item/51-54321

‪www.haprofessor.com/blog/item/51

In fact, anything written in the URL after the ID of the item (/blog/item/51) becomes a valid address that points to that post, Google scans it and actually creates hundreds of duplicate pages.

What to do???

Google will not scan it if you don't provide a link.
Furthermore you can use a canonical link extension which will solve any user input errors.

Google scans it and it's a fact.
You can not add a canonical link in K2 and you need an external plugin, which means there is a very serious bug in K2

K2 uses Joomla!'s default router.
community.joomla.org/gsoc-2017/3147-every-sunset-brings-the-promise-of-a-new-dawn-gsoc-2017.html
community.joomla.org/gsoc-2017/3147-every-sunset-brings-the-promise-of-a-new-dawn-gsoc
community.joomla.org/gsoc-2017/3147

All of these links will return content. This is Joomla!'s default behaviour.