by Asko Pesola.
I found answer to my own question! When I took backup of the course, I was just thinking that I want all the information of the course to be in the backup so I ticked pretty much everything without thinking what I am doing. I realised that you should NOT tick place "Anonymize user information". Sounds very obvious, but still I made mistake with this. So when I did not choose option "Anonymize user information", I was able to duplicate course with all the users and their work!