If i want to restrict that import data does not create duplicate records how can i achieved that in salesforce?

Hello Saurabh,

Maintaining clean and accurate data is one of the most important things you can do to get the most out of Salesforce. Use Data.com duplicate management to control whether and when users can create duplicate records in Salesforce; customize the logic that’s used to identify duplicates; and create reports on duplicates that users save.

Duplicate management features are set up and turned on by default for business accounts, contacts, and leads. To use duplicate management for person accounts, enable person accounts, and then activate person account matching and duplicate rules in Setup.

Here’s how duplicate management works.

When a user tries to save a new record, the record is first compared with existing Salesforce records to identify possible duplicates. The criteria used to compare records and identify the possible duplicates are defined by a matching rule. Next, a list of possible duplicates is returned. What happens when the record being saved is identified as a possible duplicate depends on what’s defined in the duplicate rule. For example, the duplicate rule could block users from saving the possible duplicate record or allow them to save it anyway. Both the Block and Allow options include an alert, which tells users why they can’t save the record and what to do about it. The Allow option includes the ability to report on the duplicate records.

When a user tries to save an edited record, the record is checked to see if the user has changed the value of a matching rule field. If so, the duplicate management process works as described for new records. If not, no further action is taken and duplicates are not detected.