Reproducing "The user does not exist or is not unique"

If you get this error, you may need to ensure the AD account exists:

The user does not exist or is not unique. at Microsoft.SharePoint.Library.SPRequestInternalClass.UpdateMembers(String bstrUrl, Guid& pguidScopeId, Int32 lGroupID, Int32 lGroupOwnerId, Object& pvarArrayAdd, Object& pvarArrayAddIds, Object& pvarArrayLoginsRemove, Object& pvarArrayIdsRemove, Boolean bSendEmail) at Microsoft.SharePoint.Library.SPRequest.UpdateMembers(String bstrUrl, Guid& pguidScopeId, Int32 lGroupID, Int32 lGroupOwnerId, Object& pvarArrayAdd, Object& pvarArrayAddIds, Object& pvarArrayLoginsRemove, Object& pvarArrayIdsRemove, Boolean bSendEmail)

To reproduce (in MOSS 2007):
-  Add Domain\user to AD and "Start full import" in User Profiles and Properties
- Login as user to a site collection site (assuming nt authority\authenticated users is added to default members groups)
- As user add an item to any list so that the user then appears under Peoples and Groups: All People for that site collection
- Delete user from AD and "Start full import"
- Try adding the user to a Site collection group, e.g. Members. The user will be found in the people picker but the above error occurs when you try to add user to group.
- If you re-add the exact same user (same username) to AD and do full import, the problem resolves and can add user to Members now.

Note: to avoid confusion, should probably perform "Delete Users from Site Collection" from People and Groups: All People, whenever user is removed from AD. Still looking for a way to automate this...