Double vision

Friday I received two copies of the same DVD from Netflix. Apparently this happens from time to time. Mistakes happen of course but it is interesting to speculate about what is causing this error. Is their software buggy or is there a design decision to accept an occasional duplicate mailing or is some hardware like a barcode reader malfunctioning or is it just the proverbial human error? However I don't really know enough about this sort of thing to figure out what is the likeliest explanation.