Hopefully I remember this correctly, the primary place I run into this is at work and we haven't updated past 4.4.1 yet.  There is a depends in there.  You can have duplicate username/passwords but it depends on the portal configuration.  You can share the username and password between a parent portal and its child portals, however, you cannot share username and passwords between multiple parent portals.  To share between multiple parents you will have to add a line in the UserPortal table to add that user to the new parent and that will fix the issue.  I don't think there are any commercial modules out there, I haven't looked either, I wrote a very simple one that allows you to do this exact step but you really don't need a module.  If you aren't talking about a huge number of users and portals and you have access to the database, it would probably be eaiser to add the information that is needed into the database.