Duplicate memberships created when importing

I'm trying to import a large list of contacts into civicrm. Many of the contacts are already in civicrm and have memberships associated with them. When I import the list, existing contacts who are also in the list end up with duplicate memberships.

I have duplicated this on the test instance.

Here

http://drupal.demo.civicrm.org/civicrm/contact/view?reset=1&cid=113

You can see the user 'Tim Testy'. I manually created the user and he inherited 'Site License' membership from his employer. I then imported a list of contacts, including 'Tim Testy' and set the import to skip duplicates. After the import 'Tim Testy' has two 'Site License' memberships listed