Aitor,
The issue with username uniqueness doesn't affect just JFBConnect. There was a bug in Joomla where in specific cases the username could be duplicated. We have multiple checks within JFBConnect to prevent creating them and also relied on some checks that Joomla has built in, but it seems those didn't always work.
We updated this site and found 27 duplicate usernames (out of >50,000 accounts). The last duplicate username was from 2014, so it's an issue that's progressively been going away with new safeguards implemented in JFBConnect and Joomla. JFBConnect and Joomla are our only means of registration.
With that said, can you explain how you know 'all' of the duplicate accounts were caused by JFBConnect? On our site and others with JFBConnect, we aren't seeing thousands of duplicates. You also mention JomSocial. I'd like to understand how you've narrowed down all duplicate usernames to JFBConnect. I wasn't able to understand your explanation:
i saw that JFBConnect use only the user with the first id to access from all facebook profiles,
Can you help explain this more?
Finally, Joomla has already an FAQ related to this issue:
docs.joomla.org/J3.x:Duplicate_usernames_cause_update_issue
The original issue tracker post is:
github.com/joomla/joomla-cms/issues/28308
Again, it is possible JFBConnect has created duplicate usernames. We'd like to understand your situation more so we can investigate the issue more thoroughly.
Thanks,
Alex