wp-cli/wp-cli — pull request #220 by danielbachhuber (2012-11-21 05:01:30+00:00)
Title: user import-csv: If the email address for a new user is already register...

...ed with an existing user, use the existing user's WP_User object instead.

The prior code didn't work at all because:
1) The user_login in the CSV for the new user may not have been the same as the existing user
2) add_user_to_blog() accepts a user ID, not a user_login