Actually this is done on purpose to sort of loosely support a basic requirement of Intranets ( for SSO ).  That is, if a user registers on a different portal within the same install with the same username & password... its assumed ( with an extremely high likelihood ) that its the same user.  It essentially makes that userid a member of both portals, which is what the UserPortals table is telling you.

There is nothing to fix.  If you intend for the users to be distinct folks, use different usernames... because a requirement in DNN is unique usernames across the installation ( i.e "joe" can only exist as one user in one portal... unless he's the same guy, as above ).

Scott Willhite, Co-Founder DNN

Forgive me, its been a while since I looked into this...

Bottom line is that its best for them to be separate.  My personal rule is to name the admins "admin_alias".  Bulky, but I never have a collision.  Is it the new portal create that is allowing you to do this?  Or a template?  Because it sounds like a bug... it really shouldn't take if the passwords don't match.  In a pinch, you can retrieve your passwords if you have control of your DB.... copy the password ( and passwordsalt ) from the admin account in the aspnet_users table to another user and do a "forgot password" to have it emailed to you.

btw... here' some SQL to change an existing username.  But you'll have to add the new corresponding ones in the Users table.

DECLARE @OldUsername varchar(50)
 DECLARE @NewUsername varchar(50)
 SET @OldUsername='bigwebmaster'
 SET @NewUsername='DanCaron'

BEGIN TRAN
 update aspnet_users
 set UserName=@NewUsername, LoweredUsername=lower(@NewUsername)
 where lower(UserName)=lower(@OldUsername)
 IF @@ROWCOUNT=0
  PRINT 'User ' + @OldUsername + ' not found in aspnet_users table.'
 ELSE
  PRINT 'User ' + @OldUsername + ' changed to ' + @NewUsername + ' in aspnet_users table.'
 update users
 set Username=@NewUsername
 where lower(UserName)=lower(@OldUsername)
 IF @@ROWCOUNT=0
  PRINT 'User ' + @OldUsername + ' not found in users table.'
 ELSE
  PRINT 'User ' + @OldUsername + ' changed to ' + @NewUsername + ' in users table.'
 update forum_users
 set alias=@NewUsername
 where lower(alias)=lower(@OldUsername)
 IF @@ROWCOUNT=0
  PRINT 'User ' + @OldUsername + ' not found in forum_users table.'
 ELSE
  PRINT 'User ' + @OldUsername + ' changed to ' + @NewUsername + ' in forum_users table.'
If @@ERROR=0
 BEGIN
  COMMIT TRAN
  PRINT 'username change committed'
 END
ELSE
 BEGIN
  ROLLBACK TRAN
  PRINT 'username change rolled back'
 END
GO

Scott Willhite, Co-Founder DNN