I exported the users from one portal and imported them to a second portal. If you search for users (in the DNN admin user accounts screen) by username, you see a user once. If you search by last name, you see the user twice.

Did I do something wrong in the export/import? In my Excel spreadsheet, the user is in there only once.

If you go to edit either user, they have the same user id.

DNN 5.1.4 and MMS Pro 1.3.74.090612_dnn5100

The import uses core dnn for creating accounts and duplicates are either logged to exceptions file or imported with suffix. It is not possible to import the same user twice.

Are both portals on the same DNN instance ?

yes they are. I was using the MPUS module to share the users, but we want to upgrade our DNN instance, move the users and then delete the old portal.

You should turn the sharing off, export users from A, delete users from A or even better delete the portal and import users to B
Or even better do the import on new DNN instance to have clean start.