IMEI issued to more than 1 device?

I have had the S4, S6, S8, S10 and now the S21 ultra. I have generally been happy with these phones. The latest purchase has a promotion for a trade in of one of my old devices with a guaranteed trade in on R4800. I have tried without success to process the trade in. The system gives me an error with the IMEI number of the new device being invalid. I have called into samsung and the agent told me that the IMEI was allocated to bith a S21 ultra and another device. When I used the online chat function, the agent didnt understand how the R4800 tradein worked. Samsung tried to call me and their lines didnt work as I couldnt here anything form the caller. The tradein promotion ends on 31 July with no mechanism for me to resolve this issue. Dissappointed that i have a device with a dubious IMEI number and that i cannot resolve this.