The error message "Use one of these records?" indicates that the user may be attempting to create or update a record that already exists on Salesforce. This error is system-generated by Salesforce and is triggered by the duplication prevention settings that have been configured in the Salesforce instance connected to your portal.
Review the object's duplicate rules settings on Salesforce and disable them if necessary. Keep in mind that additional duplicate rules may also trigger this error
An alternative course of action would be to exclude the Salesforce integration user from the Salesforce duplication rule. This would allow portal users to submit duplicate records without being subject to the existing duplication prevention settings.