It often happens to me that the till says that the receipt has already been redeemed. But I've never used the receipt since I received it. I don't use them digitally, only the physical ones. I've already lost a good 4 2x vouchers thanks to this message...
I had exactly the same case at the end of 2024. After a written request, they claimed that I had used the vouchers for an online purchase from ExLibris. But that's not true. The vouchers were never activated beforehand, they were just stored on my desk. My consequence: I no longer use Cumulus, despite my 35 years of employment with Migros!