Open the contact that has been linked. Browse to the bottom where it says 'Joined Contacts'. Tap those icons and you will be in a screen where it lists all the contacts it was joined from. There are '-' icons next to each contact. Tap the icon will remove the link.
After they're unlinked, you can delete the duplicate entry. These multiple contacts are produced when various social apps like twitter and facebook provide the same email address listed in your contacts. The SETTINGS/ACCOUNTS AND SYNC menu gives you options to turn them off. Perhaps, your entries are not linked and you have separate listings for the same contact from different sources - you can link them to merge into one entry.