But if the crew were expecting to fly to DUS and were present with a FPL that said EDI, they must surely have questioned it with someone before engine start.
The point is that we don't know what was or what wasn't said. Obviously the crew were somehow or other convinced and not particularly surprised to be going to EDI, only the passengers! As ATC Watcher says, a lot of holes in the cheese must have lined up.
Not too dissimilar to this I remember a day I was on duty when we questioned an aircraft's unexpected turn over a way point and questioned their intentions. They replied that they were following their flight plan route. We asked confirmation of their destination and discovered it was different to the one in the FPL we had received. In this case it turned out that the crew got one set of paperwork (the correct set) but that their ops had sent an incorrect FPL to ATC. Of course the SID and initial routing was the same until they got to that way point in our airspace and only then was the discrepancy noticed. So, these things happen.