A processing method under data duplication, applied to a communication system, wherein the communication system comprises a first communication device comprising a packet data convergence protocol, PDCP, entity, the method comprises: duplicating, by the PDCP entity of the first communication device, target data when data duplication is enabled, wherein the target data comprise data delivered by an upper layer of the PDCP entity and received by the PDCP entity of the first communication device, which are not transmitted from the packet data convergence protocol, PDCP, entity to a radio link control, RLC, entity, except data that are being in Automatic Repeat-reQuest, ARQ, transmission; or except data of which at least one segment is being in Automatic Repeat-reQuest, ARQ, transmission; or except data that are being in Hybrid Automatic Repeat reQuest, HARQ, transmission; or except data of which at least one segment is being in Hybrid Automatic Repeat reQuest, HARQ, transmission; or except first data, PDCP COUNT of the first data is less than or equal to PDCP COUNT of data that are being in Automatic Repeat-reQuest, ARQ, transmission; or except second data, PDCP COUNT of the second data is less than or equal to PDCP COUNT of data of which at least one segment is being in Automatic Repeat-reQuest, ARQ, transmission; or except third data, PDCP COUNT of the third data is less than or equal to PDCP COUNT of data that are being in Hybrid Automatic Repeat reQuest, HARQ, transmission; or except fourth data, PDCP COUNT of the fourth data is less than or equal to PDCP COUNT of data of which at least one segment is being in Hybrid Automatic Repeat reQuest, HARQ, transmission; and delivering, by the PDCP entity of the first communication device, duplicated target data to the radio link control, RLC, entity, wherein being in transmission comprises that the radio link control, RLC, entity does not completely receive the duplicated target data correctly sent by the packet data convergence protocol, PDCP, entity, and the radio link control, RLC, entity does not send acknowledgement information to the packet data convergence protocol, PDCP, entity; characterized in that a PDCP COUNT value is a counter of PDCP, the COUNT value comprises a hyper frame number, hfn, and a sequence number, PDCP SN, corresponding to a data, when the PDCP entity delivers one PDCP data at each time, the PDCP COUNT value will be increased by 1, a PDCP COUNT value of certain data being less than a PDCP COUNT value of a data being in transmission indicates that the certain data have been delivered; a PDCP COUNT value of certain data being equal to a PDCP COUNT value of the data being in transmission indicates that the certain data is being delivered. duplicating, by the PDCP entity of the first communication device, target data when data duplication is enabled, wherein the target data comprise data delivered by an upper layer of the PDCP entity and received by the PDCP entity of the first communication device, which are not transmitted from the packet data convergence protocol, PDCP, entity to a radio link control, RLC, entity, except data that are being in Automatic Repeat-reQuest, ARQ, transmission; or except data of which at least one segment is being in Automatic Repeat-reQuest, ARQ, transmission; or except data that are being in Hybrid Automatic Repeat reQuest, HARQ, transmission; or except data of which at least one segment is being in Hybrid Automatic Repeat reQuest, HARQ, transmission; or except first data, PDCP COUNT of the first data is less than or equal to PDCP COUNT of data that are being in Automatic Repeat-reQuest, ARQ, transmission; or except second data, PDCP COUNT of the second data is less than or equal to PDCP COUNT of data of which at least one segment is being in Automatic Repeat-reQuest, ARQ, transmission; or except third data, PDCP COUNT of the third data is less than or equal to PDCP COUNT of data that are being in Hybrid Automatic Repeat reQuest, HARQ, transmission; or except fourth data, PDCP COUNT of the fourth data is less than or equal to PDCP COUNT of data of which at least one segment is being in Hybrid Automatic Repeat reQuest, HARQ, transmission; and delivering, by the PDCP entity of the first communication device, duplicated target data to the radio link control, RLC, entity, wherein being in transmission comprises that the radio link control, RLC, entity does not completely receive the duplicated target data correctly sent by the packet data convergence protocol, PDCP, entity, and the radio link control, RLC, entity does not send acknowledgement information to the packet data convergence protocol, PDCP, entity; characterized in that a PDCP COUNT value is a counter of PDCP, the COUNT value comprises a hyper frame number, hfn, and a sequence number, PDCP SN, corresponding to a data, when the PDCP entity delivers one PDCP data at each time, the PDCP COUNT value will be increased by 1, a PDCP COUNT value of certain data being less than a PDCP COUNT value of a data being in transmission indicates that the certain data have been delivered; a PDCP COUNT value of certain data being equal to a PDCP COUNT value of the data being in transmission indicates that the certain data is being delivered.