I'm using the Migrate module to migrate users from a non-Drupal PHP/MySQL site to Drupal 7. The users are spread across two tables, the second of which has a non-unique email column, many values of which are present in the first table.
Generally, how do I import the second table, inserting users with new email addresses, and updating a couple fields whenever the email address is already present among the users created from the first table and avoiding duplication errors from the second?
The users from the first table are administrators. The users in the second table are clients. There are other tables that references administrators and clients by ID.
The ID columns for the two types of users are unique only within its respective table.
I made all the client uid's globally unique by adding 10,000 to all their uid's and updating their references in the other tables accordingly.