Migrate users from a table with non-unique email column
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.