You have to ask directly in your order that you want imperatively different serial numbers.e longer because they will have to repatriate discs in their digérants shop and to sort out according to your request but as you wrote it it is possible that you already doubt the answer now it is up to them to satisfy the customer.. or not ...

If you place the order once via Galaxus and once via a retailer. That is indeed possible.
However, even if Galaxus has 100 plates, this means that different batches can be delivered thanks to the chaotic storage system.
Direct influence, but not possible

Since when do different hard drives have identical serial numbers? In my case, each hard disk has a different S/N.

There may be different batches (mine all had the same production date) but I doubt whether Galaxus selects them.