Hello Vagelis Kretsimos,
It seems that this behavior is omission on our side. It seems that when a page is duplicated, it is with status New instead of status Published which is normal when the workflow is switched on. As a workaround for the time being, you could open each duplicated page and click on the save button. It should do the trick.
The bug is added to our to do list and will be fixed for the next release. Your Telerik account has been updated for reporting this.
All the best,
the Telerik team