You can import a backup file of your contacts or revert your contacts if you are syncing them in another source.
If you created a Backup File in Sync.ME:
If a Backup file was created before the merge, you can import this Backup File into your device. To do this: From your phone, open up the email that you sent yourself with the Backup file. Open the Backup file to import the contacts back into your device. Please note that this will add the contacts into your device which may bring more duplicates. To prevent this you can clear the contacts from your device (create another Backup file of your contacts before doing so).
If you sync your contacts with Google:
You can revert your contacts back to a specific date, before they were merged. Here are instructions from Google about how to do this: https://support.google.com/mail/answer/1069522?hl=en
Restore iCloud contacts:
1. First make sure your backup is intact and available
2. Go to: Device Settings>iCloud>Contacts (Off)>Keep on my iPhone
3. Now turn Contacts On>Merge.
Recovering contacts from iTunes:
The following link shows how to restore contacts through iTunes: http://bit.ly/1mPrjKI
How to create a backup in iCloud:
1. Sign into iCloud
2. Contacts> Settings (bottom right)>Select All> Export vCard. This creates a file for all of the selected contacts in VCF format.