This one got me puzzled. I used to be able to use the same account+password combination to login in several portals within one dnn installation. However, when I tried to replicate this for other users, it did not work. I never researched this, because it had low priority. The other users had to use a different account name and that worked around the problem.

However, since the upgrade to dnn 6.2 (06.02.02 actually) the issue is back. I tested successfully to login into one portal using my Facebook account, but when I try to use this account for a second portal within the same dnn installation, dnn refuses with the message: "A User Already Exists For the Username Specified. Please Register Again Using A Different Username."

These are two portals with the same kind of content (news in Khmer) and users who use their Facebook account for one web portal may very well want to do the same for the second portal. I cannot figur out why sometimes this works and on other times it fails.

That's what I thought with the failed manual registrations. My registration in several portals had succeeded, so I figured the others had their password wrong the second registration (though I never checked it out).

But using login with facebook account, I don't have to give accountname and password. Facebook does the authentication and sends a code back when login succeeded. This code indeed exists in a different portal but should be accepted in all portals, isn't it?

I can confirm this behavior on a new dnn installation. It has only 3 user accounts: the host account, a FB account on site 1 with admin role (for the admin indeed) and a FB account on site 1 (mine).

Now I cannot login in site 0 with the same facebook account.

I do not believe this by design. I think this is an issue that went unnoticed (since around 5.2 or 5.3) and that becomes accute with the FB login.