The duplicate detection rules define the entities and fields to compare when performing duplicate detection. Each rule defines the entity that wants to find duplicates. The rule then has one or more fields to match together with the criteria for matching values.
Clicking on Duplicate detection rules in the Power Platform admin center opens a new page showing the list of rules already in the environment, as shown here:
There are rules for Account, Contact, and Lead included. You can edit these rules and create new rules. To create a new rule, click on New in the toolbar. This will open a new window, as shown in the following screenshot:
The duplicate detection rule first defines the entity, or record type, to check duplicates against. Select the entity from the drop-down list. The entity will automatically be selected as the matching record type.