A processing method under data duplication and related devices are provided, which are applied to a communication system. The communication system comprises a first communication device, and the first communication device comprises a PDCP entity. The method comprises: when data duplication is enabled, the PDCP entity of the first communication device duplicates target data, and the target data do not comprise data of which transmission have been completed and/or data that are being transmitted; and the PDCP entity of the first communication device delivers the duplicated target data to the RLC entity.