3 years ago I got duplicate mile on a trip to Italy thru Brussels on Sabensa/Delta.
I am not sure how it happened, but I got creidt twice for ther same flight.

I was flying into Itlay and had originally made a reservation on Delta. I had then bought a ticket thru Sabena on the same flight because it was cheaper. When I checked in at Atlanta I mistakenly went to the Delta international check in and they were not busy and checked me in anyway.

Somehow in all this I got credited twice on one leg of the trip over, not the flight from Bru into italy but the flight over.

I wish I knew how to duplicate it, I would, having an addiction to miles that drives me to all kinds of immoral acts.