It occurs when a single digital token is used several times since the token is typically made up of a digital file that can be easily cloned. It simply causes inflation, and businesses are forced to take a significant loss. One of the main goals of Blockchain technology is to eradicate this method as much as possible.
Blockchain avoids double-spending by requiring several parties to validate a transaction before it is written to the ledger. It's no exaggeration to claim that bitcoin's entire structure of Blockchain, mining, proof of work, complexity, and so on exists to create this history of transactions that is computationally impractical to change.