We are in an Exchange 2010 SP2 RU6 environment running Outlook 2010 Version 14.0.6137.5000
Support has been able to reproduce.
User A has an Auto-Complete entry for User X who is listed in the Global Catalog. (Secretary)
User B has a manually created Outlook contact for User X.
User C has a manually created Outlook contact for User X.
User A has Editor permissions for User B and User C.
User A deletes the contact for User X from User B's contacts.
User A copies the User X contact from User C to User B's contact list.
When User A tries to send User X and email, they are no longer in the Auto Complete list.