For example, the "email" field is usually unique to prevent repeated emails from getting in your list when you import contacts, manually add them or use a sign-up form in your site.
Can I turn any field in my contact list into a unique field?Yep. Say you've already created a field named "Customer code" which you want to be made unique so it can only include different codes:
1) Go to the "Lists" menu, head over to the list containing that field and hit "Options".
2) You'll be taken to the list settings page. Scroll down to "Unique fields", click "Add", choose the field to be set as unique and save your changes.
Done! From then on, E-goi will check for duplicated content in that field any time you add someone to your contact list.