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In this paper, a two-user superposition modulated cooperative relay hybrid automatic repeat request (HARQ) system is proposed. Two mobile station (MS) uplink communications to the base station (BS) with cooperative assistance of a relay station (RS) are considered. The RS is assumed to be able to perfectly monitor and decode the two MS-to-BS transmitted packets and assist the two MSs by immediately transmitting a cooperative superposition modulated packet that combines the two MS generated packets in the analog domain. The BS is assumed to have perfect channel statistical information and can feed back the appropriate superposition weighting factor to the RS regularly. The HARQ retransmissions, if needed, are then jointly performed by the RS for both MSs instead of separately for each MS. The superposition of the modulated packets is repeatedly retransmitted by the RS to the BS if needed. With the foregoing setting, we derive an information-theoretic achievable rate region and an approximation of the system outage probability. Based on the high SNR behaviors of the approximated system outage probability, we obtain the superposition weighting factor using a simple and efficient searching algorithm. We also analytically derive the HARQ system throughput as the performance metric for comparison. Compared with the traditional relaying HARQ system, our proposed scheme exhibits more than 30% gain in terms of system throughput, depending on the fading statistics of the RS-to-BS and MS-to-BS channels.