Duplicate Detection Rules lets you define rules to check for duplicate records based on the duplicate detection settings. We can select fields that we want to validate for duplicate detection. It is always recommended to use a unique field for duplicate detection checks, such as record ID or email address.

The following screenshot shows the example Email address field check rule, which will fire based on duplicate settings such as on record creation or on updating of an account entity:

This will show a duplicate detection dialog if duplicates are found. Keep in mind that even though duplicate detection is set up, it does not stop users from creating duplicate records; so, Dynamics 365 CE users can still create account records with email records that match those from existing account records. If we want to prevent users from creating duplicate records, we can register custom logic in a Dynamics 365 CE organization. Once a duplicate detection rule is ready, we can publish it. Only published duplicate detection rules are used by Dynamics 365 CE for checking duplicate records.