You will want to start off by making a backup of your contacts.
Select All Contacts.
Click a contact in the Contacts list.
Choose File > Export > Export vCard or choose Edit > Select All to select all cards for export.
Choose where you want to save the files, then click Save.
Merge duplicate contacts.
From the Card menu, select Card > Look for Duplicates.
When asked, click Merge.
Repeat steps 2 and 3 until no duplicates are found.
You can also find an article here on komando.com which suggests an app that can clean up your contacts as well.