One of the original intentions of blockchain design is to prevent and resist double-spending attacks. Double-spending attacks refer to the problem of funds used once being used multiple times, which is usually a challenge in traditional digital currency systems. Blockchain solves this problem through consensus mechanisms and decentralized networks. However, in theory, double-spending attacks are indeed possible under certain conditions, and the main ways include 51% attacks: 1. **51% attack**: If a malicious entity controls more than 50% of the computing power (under the PoW consensus mechanism) or more than 50% of the equity (under the PoS mechanism) in the blockchain network, the entity may be able to modify transaction records, including double-spending. This is very difficult on most large blockchains, such as Bitcoin and Ethereum, because it requires huge resource investment, but it may be relatively easier to achieve in some smaller blockchains. 2. **Timestamp attack**: Influence the consensus mechanism of certain blockchains by modifying the timestamp in order to create a forked chain on which double payments are made. But this requires sufficient control over the network. 3. **Competition attack**: Make multiple transactions that consume the same coin in an unconfirmed transaction, hoping to get users to accept the unconfirmed transaction before one of them is confirmed. But modern wallets and software have largely reduced such risks. Although these attacks exist in theory, most blockchain network designs have strict security measures to defend against these attacks, such as block confirmation, monitoring post-forking, economic penalty mechanisms, and the establishment of a decentralized node network. In order to effectively prevent double-spending attacks, the following measures are recommended: 1. **Avoid using smaller and less secure blockchains**: Large blockchains are generally more secure because of the large number of network nodes. 2. **Wait for multiple block confirmations**: When making large transactions, wait for enough block confirmations to increase the irreversibility of the transaction. 3. **Use strong wallets and exchanges**: Choose service providers that have been evaluated and audited by the industry. Although double spending is a core problem that blockchain must solve, most mainstream blockchains are still very reliable and secure through strict security measures.