Global Address List Contacts do not appear in the list of contacts when using "Add to Outlook" or when connecting phones.  Therefore contacts that already exist in the GAL must be entered again resulting in duplication.

I will also add - as the Global Address List contacts are available to the entire organization, some information may need to be kept private.  For example, you might want your boss to have your cell number, however not the entire organization.  When you open a contact from the Global Address List a user only has read only access.  It would be useful if they were able to have an Add To function - which is stored only in that user's contacts.  For example, the Boss could add the employee's cell phone number, only visible to him/her.