When configuring a duplicate detection rule in Power Apps with Microsoft Dataverse, to automatically delete duplicate records during a data import, you should enable the "During data import" option. This option allows the detection rule to automatically delete duplicates as they are imported into the system, without requiring user intervention.
Reference:
https://docs.microsoft.com/en-us/power-platform/admin/create-duplicate-detection-rule
https://docs.microsoft.com/en-us/power-platform/admin/data-integration-duplicate-detection