Massive nodes in a blockchain form an off-chain distributed storage network to provide storage resources for users to meet large data upload requirements. However, this storage approach introduces security and performance issues. Firstly, it is difficult to guarantee the integrity of the data uploaded, and these data may be easily corrupted or lost. Moreover, uploading excessive duplicate data leads to a waste of storage resources. In this study, to address these issues, with a double-copy storage model for blockchain off-chain storage, a novel public auditing scheme with client-side deduplication is proposed to reduce the storage overhead of nodes and check the integrity of the off-chain data. Based on smart contracts, our scheme could realize efficient user ownership and off-chain data integrity verification automatically. In addition, both data encryption and deduplication are achieved based on message-locked encryption and an improved authenticator generation algorithm. Security analysis and experimental comparisons show that the proposed scheme is effective and practical.