Hello ycchan
The ticketing system does not check whether you have already purchased the same ticket. It would not even possible when you purchase as a guest. Did you by any chance use both tickets in such a way that you did show one ticket at the first ticket control, and at the next check, the other ticket? In this case, both tickets do have a “used & checked time stamp” to which SBB refers.
Please contact SBB again using the complaint form, attach both tickets or mention their ticket IDs and describe the case:
https://www.sbb.ch/en/help-and-contact/customer-service/praise-comments/claims-complaints.html
The SBB Community is a customer help forum run by other customers. We are therefore unable to assist you in this matter other than giving the link to the customer complaint form. Nevertheless, we do like a hello and a thank you as part of a question, as we are all volunteers.
Kind regards,
Roland