I’m afraid you’re stuck now. Maybe your device was returned by another customer after they already registered it; maybe you can get a new device from your supplier. I guess the TTN crew (like @htdvisser) technically could delete the existing application, but how can they be sure that the other user who registered that AppEUI doesn’t actually have a device that also uses that very same AppEUI? Not all devices send data very often.
Is there any chance you can change the AppEUI that is in the device? This was a solution for bad AppEUIs in LHT65 devices, but that needs the device to allow for such changes.
Just in case it’s important: what did you mean by the first line in the following?