As a system for detecting double payments occurring in the blockchain, the structure of a plurality of blocks constituting the blockchain is analyzed, and a branch search unit searching for a branch generated in the blockchain, and when the branch is found, forming the searched branch A block determination unit for determining a first block and a second block, a comparison unit for comparing transaction information included in the first block and the second block, and determining whether a double payment has occurred in the blockchain, and a double payment generation determination unit. When a double payment occurs on the blockchain, a warning message transmission unit for transmitting a warning message to nodes that share a plurality of blocks constituting the blockchain.
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