Double-spending is a major problem in cryptocurrency ecosystems. When a user spends a particular cryptocurrency, standard network security protocols will prevent that same cryptocurrency from being spent again.Double-spending occurrences can happen due to misusing or manipulating a blockchain's code, or due to malicious third-party code exploiting blockchain weaknesses.

The 51% attack is one type of double-spend attack that attempts to gain control of more than 50% of the hashing power of a blockchain network. By doing so, a malicious actor is able to control the network and make invalid transactions such as sending the same cryptocurrency twice. To protect against such an attack, cryptocurrency networks use mining pools, where miners pool their resources together to increase the hashing power of the network.

The unconfirmed transaction attack is another type of double-spending attack which does not require gaining control of the majority of the hashing power of a blockchain. It works by broadcasting two conflicting transactions to the network, meaning that the same funds are being sent to two separate addresses. The attacker then hopes to convince miners to include the transaction with the higher mining fee in the blockchain, which annuls the other transaction.

Cryptocurrency users are also vulnerable to double-spending through stolen funds, either through hacking or breaching the securities of the wallet. It is for this reason that it is essential to store cryptocurrencies in a secure wallet, certified by industry professionals and thoroughly tested against attack scenarios. Additionally, fraud analysis and identity verification tools can decrease the risk of fraudulent double-spending.

To sum up, double-spending is a serious risk to cryptocurrency networks. With the advent of more sophisticated cryptocurrency technologies, the window for double-spending is closing as miners and network operators are becoming more vigilant about protecting against malicious actors. In order to protect against double-spending, blockchain networks should maintain high levels of security and users should remain vigilant by using secure wallets and fraud analysis tools.