First thing you should do is verify that your contacts in outlook sync with any other device you have contacts on. You can do this with MailSync+ or other similar solutions.
Now open outlook and select your contacts folder, then changing to list view will make this much quicker.
- Select a duplicate entry and use Ctrl+C (to copy the entry)
- Select the entry you would like to merge the duplicate into, use Ctrl+V and it will tell you of any differences. At which point you can choose to accept the changes or cancel.
- Go back to the duplicate and use Del to remove it.
That's it you're done.
you can also use the import merge method for bulk duplicate entries.