The problem of double-spending is one of the main worries for any cryptocurrency developer. This refers to the occurrence of a person using a balance of a particular cryptocurrency more than once, effectively leading to a discrepancy between the spending history and the quantity of that cryptocurrency that is actually available as well as how it is allocated.

Cash does not have the problem of double-spending; if you pay for a commodity with a $10 bill and give it to the person who sells it, you cannot use that same $10 money again to buy something else.

However, a transaction made with a digital money like bitcoin only happens online. This indicates that it is possible to replicate the transaction details and rebroadcast them, allowing a single owner to spend the same amount of BTC several times. A technical issue that arises with the notion of a digital currency is the ability for somebody to duplicate the digital money and spend it simultaneously at two or more places.

This 'double-spend' problem is prevented in blockchain-based cryptocurrency assets, such as Bitcoin by using a consensus mechanism known as proof-of-work (PoW). This PoW is carried out by a decentralized network of 'miners' who not only secure the fidelity of the past transactions on the blockchain's ledger but also detect and prevent double-spending.