I have also seen this. The two cars were parked on a private driveway so, technically, the owner(s) were not committing any offence unless the car with the false number plates (possibly both of them) was driven. I reported it to the local nick who said they were �taking an interest�. A few weeks later when I passed the same area, the cars were gone, and the house was empty.
There is no doubt that two cars cannot have the same registration mark. I would start off with the local nick as, in my experience, the DVLA are not particularly good at �enforcement� unless there is money to be made.