Question: We've encountered cases with our customers seeing the following: errorMessage=blacklist token already exists when trying to submit their payment.
Answer: Your customer is getting the error because the token has been blacklisted. Every page submission consumes a token. Once a token have been consumed, it is blacklisted for 48 hours for security and fraud prevention. When a Hosted Checkout Page needs to be reloaded, for example, after an unsuccessful submission, a new token must be created to generate a new iFrame.
You can only reuse a token after 48 hours. The best practice is to re-generate another token when submitting the Hosted Page. Please refer more details at Generate the Signature for the iFrame.