You will need a package editor to alter the xml file, either
Nraas Packer or s3pe.
Personally I normally use s3pe found here
http://www.den.simlogical.com/denforum/index.php?topic=189.0Just start it up and open the name list tuning package, you will see two files a key and an xml, right click the xml and choose notepad, now you can edit it with your new names.
Once done save the notepad file and close it, it will prompt you to commit changes, say yes then just save the package file and it's ready to use.
Or alternatively use Nraas Packer, just open the tuning file in that then right click and edit, it shows the xml at the bottom of the window where you can add stuff. It's a much simpler interface than s3pe so you might find it better just for changing a tuning file.
I don't know what would happen with duplicate names, it might as you say make them more common as I assume the game randomly picks from how ever long the list is regardless of what the actual names are on it.