How to enable duplicate prevention of records?

1. Click on  > All Settings > Module Manager > Module Layouts & Fields.​​​​​​
2. Select the particular module from the drop down.
3. Click on Duplicate Prevention and Enable Duplicate Check option.
4. Select the fields from the drop-down, on which you want to detect the duplicates.
5. Sync Actions.
6. Click Save.

Refer this document for detailed information on duplicate record prevention and handling duplicate records.