There can be a possibility that the holder of the digital token can copy it and send it to a merchant while keeping the original to himself.
Double Spending is the risk that a digital currency can be spent twice.
However, Bitcoin has a mechanism based on transaction logs to verify the authenticity of each transaction and prevent double-spending.
Since the Blockchain comprise of blocks which contain each transaction history, double spending is avoided. This mechanism ensures that the party spending the bitcoins really owns them.
The rules by which Blockchains are governed by the miners. If a group of miners who control more than 50% of the network’s mining hashrate, or computing power collaborate, they can make unethical rules, reverse transactions, could double spend the coins. Such an attack is referred to as 51 % Attack.
Krypton and Shift, two blockchains based on ethereum, suffered 51% attacks in August 2016.