For one of our Drupal 7 Site, we are importing data using the Feed importer module. Drupal feeds module provides the possibility to make SOME fields as unique fields when importing content from a file(such as the Title field of a node for example). In my case, I need to configure other fields as unique. A simple Drupal contributed module will help to do the same and its the Field Validation module.

How to Configure  the Field Validation module

• Step 1: Download and enable the Field Validation module
• Step 2: Also Enable the Field Validation UI module that comes bundled with the Field Validation module
• Step 3: Go into Structure > Field Validation and add a new rule, with the following parameters (for the validator, select "Unique Values" and for the scope, select Bundle or Entity depending on what you need)

• Step 4: Go to Feeds Importer configuration, in the Mapping of the Entity: you will now be able to select the field we need to make it Unique.

Hope this helps! Please feel free to get in touch with us for further queries.