"Double spending" refers to when Bitcoin that has already been used is used again. In Bitcoin transactions, Bitcoin can only be used with the signature of the person who has the private key, which prevents unauthorized use. However, this does not prevent the person with the private key from using the same coin in two transactions. By using a blockchain and "proof of work," Bitcoin is set up to prevent double spending.
Even if there is no malicious intent, normal transactions may be subject to double spending due to an attack called a "Race Attack." However, in this case, a semantically identical transaction is generated, so it does not pose a large risk.
The transaction created based upon double-spent Bitcoin will become invalid, so you should be sure that the Bitcoin you use has six authentications.
It is possible for malicious programmers to intentionally double-spend Bitcoin. So, it is important for operators to have the means to prevent double-spending.