Migrate users from a table with non-unique email column 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.
It depends on what kind of method, modules, tools you'd like to use to migrate the data as each of them might have their own settings for the kind of situation you had described.

If it's just a custom script migrating data between databases, then I would treat all users the same and import any of the tables first and then import the second one ignoring the error message - Drupal won't be importing users with the existing e-mail twice. After the migration you could go over each admin to assign appropriate role.