This tag should be used for questions related to double spending. Double spends are when a money is spent more than one time. This is considered a form of fraud.
In Bitcoin, double spends are impossible. A transaction that spends money already spent in another transaction will never be valid and confirmed. However unconfirmed transactions can conflict by spending money spent by another unconfirmed transaction. Double spends are often used to defraud merchants by allowing a customer to not pay the merchant. The risk of double spends is the primary reason that merchants wait for one or more confirmations before accepting a Bitcoin transaction.