At receiving side RLC there is no need of knowing whether the received RLC pdu is newly transmitted or retranmitted.
Whatever the RLC PDU it is, receiving RLC treats in the same way.
If receiving RLC doesn't receive the RLC PDU(s), Receiving includes NACK_SN(s) for corresponding missed RLC in the STATUS PDU and send to transmitting RLC. Based on this STATUS PDU, transmitting RLC considers RLC PDUs which are for retransmission, and performs retransmission of those RLC pdus until the retransmission count for a RLC PDU reaches maximum number of retransmissions provided by higher layers.
