From my experience, here is how it works (at least in DNN 4.0.3, have not upgraded to 4.3.x at this time).

The same user name may exist in multiple portals (in my case different parent portals within one DNN instance), however there are two caveats.

1. The username and password must be identical on all portals
2. The user must be authorized on one portal before attempting to register at another portal or they will receive an error message indicating their user name is invalid.

I am not certain if they must be authorized only once before registering at all other portals, but I think that is the case. Just haven't tested to that degree.

Rob Ralston

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Rob Ralston, SilverBullet Technologies LLC, www.silverbullettech.com

Passwords are 99.999..% gaurenteed to be different, as they will be two different people, quite probably in different parts of the world. I suppose there is some chance these two people somehow choose the same password, but extremely unlikely