The was topic ask if Joomla 1.8.3 is compatible and we have installed the 1.8.3 with success.

You error message seems to show that you try insert a duplicate key into the community_users table.
I don't know if the website where you try to upgrade Jomsocial is a website that share JomSocial or the website where JomSocial is physically installed.
If this is a slave site that share JomSocial present on another website, then you may perhaps have to uninstall and re-share JomSical to take in effect the possible database layout modifications.

If this happen on the website where is physically installed JomSocial then I don't know.
Perhaps I could find the reason if I connect on your environement but in this case, this will be with our billable support that you can order at
www.jms2win.com/download?page=shop.produ...35&category_id=1