In the Professional and Enterprise Editions, you can use the Skip or Overwrite option to avoid creation of duplicate records while importing. These options can be used only if the import file is in CSV format with not more than 5000 records.
You can also avoid import of duplicate records based on the email id.
Select Setup > Customization > Layouts
Select the desired module from the drop-down list.
Scroll down and select the "Do not allow duplicates in Email field" checkbox.
Click Save. When you import a set of records, Zoho CRM now matches the email id in your import spreadsheet with the email ids already existing in the database. If it finds a match, it prevents the import of the record corresponding to the duplicate email ID.