The use case is when user is encountering the following error when configuring SAML on his NetSuite account.
The entity ID is a unique identifier for a SAML configuration set up for a NetSuite account. To determine the entity ID currently saved in one's NetSuite account, the user must login to his NetSuite account with the role that has the 'Setup SAML Single Sign-on' permission and navigate to Setup > Integration > SAML Single Sign-on.
To avoid the issue, the user must go to his Identity Provider configuration and modify the entity ID to make sure that it's unique. After that, the user has to delete the IDP saved in his account by clicking on More Actions > Delete IDP Configuration and re-upload the newly-downloaded IDP metadata.
Another possible scenario for this is that the user might have two Sandbox accounts and he has already used the same IDP in one of his Sandbox accounts. If this is the case, the user has to contact his chosen Identity Provider to acquire another IDP configuration.