The system did not make mistakes.
If you are being told that you have previously completed the questionnaire then there is nothing additional that you need to do at this time.
If you are instructed to try and complete the questionnaire again, let the person telling you this know that the system tells you specifically not to do so,
Creating a new account to complete the questionnaire again will result in your accounts being flagged and locked as a Violation of the T&cs (terms and conditions) and your results will not be available to the company.
If you have additional questions, please use the contact method found on the page.