An operating method and an apparatus according to data duplicate retransmission in a mobile communication system are provided. A method of a User Equipment (UE) according to data duplicate retransmission in a mobile communication system includes storing a Media Access Control Protocol Data Unit (MAC PDU) received from an Evolved Node B (ENB) in a soft buffer, decoding the MAC PDU, determining whether the decoding is a first successful decoding of data of the corresponding soft buffer, and determining whether to forward the decoded MAC PDU to an upper layer according to the determination result.