Cross-user deduplication is an emerging technique to eliminate redundant uploading in cloud storage. Its deterministic response indicating the existence of data creates a side channel to attackers, which makes the privacy in the cloud at risk. Such kind of attack as well as further appending chunks attack, still cannot be well resisted in current solutions, thus is becoming a big obstacle in using this technique. We propose a secure cross-user deduplication, called Request merging based deduplication scheme (RMDS), which takes the lead to consider resistance against appending chunks attack in a lightweight way, let alone side channel attack. We utilize the proposed XOR based chunk-level server-side storage structure together with a request merging strategy to obfuscate attackers in minimized communication overhead. The experiment results show that, with security guaranteed, the proposed scheme is more efficient comparing with the state of the art.