Adding Twitter account in ML doubles up contact entries

When OS X Lion introduced fields to store Twitter account names in Contacts, I updated my contact lists accordingly. However, with the new Twitter integration in Mountain Lion, when I added my Twitter account to the Mail, Contacts & Calendars section of System Preferences and told it to update my address book, contacts that I had updated previously now have two Twitter entries in their contact details - a lowercase "twitter" and a capitalised "Twitter"; the former presented as a clickable link e.g. twitter:@davidkeaveny which takes me to the contact's details in the official Twitter client; the other does nothing. Just to be clear - this is duplicate fields in one record, not duplicate records.

Obviously only a minor annoyance, but does anyone have any idea why Contacts had to create a new set of fields for Twitter, and not just update the existing details?