How to enable duplicate prevention of records?
- Click on > All Settings > Module Manager > Module Layouts & Fields.
- Select the particular module from the drop down.
- Click on Duplicate Prevention and Enable Duplicate Check option.
- Select the fields from the drop-down, on which you want to detect the duplicates.
- Sync Actions.
- Click Save.
Refer this document
for detailed information on duplicate record prevention and handling duplicate records.