Configure duplicate records.
Go to Setup > Administer/Data Management/Duplicate Management > Duplicate Rules.
Click Standard Contact Duplicate Rule.
select the checkbox for Active field.
Note
This ensures only the Dealing with Duplicates section in Contact Routing works
In the Mapped link, complete the fields based on the match percentage you need (refer to field mapping in Salesforce):
For a 100% match, complete all the fields.
For a 50% match, complete at least half of the fields.
