To check for duplicates in Global Address book :
Go to Common -> Global Address book
Select a record and click on “Checkfor duplicates”
Select the values to include in the duplicate search (like name, state, zip code, phone, etc..) and press search
The duplicated records will appear in the below grid
If you want to merge the 2 records :
On the Action Pane, on the Global address book tab, in the Maintain group, click Merge records.
In the Merge party records form, in the Select the party to retain field, select the record that you want to keep. Information from the other selected records will be merged into this record.