Harry had once before had the experience of seeing something that Ron could not, but that had been a reflection in a mirror, something much more insubstantial than a hundred very solid-looking beasts strong enough to pull a fleet of carriages.
Harry Potter and the Order of the Phoenix
I'm not quite sure how to parse "Harry had once before had the experience..." correctly. Maybe, it could be paraphrased as: Harry had once before he had the experience.... But it looks a bit odd. How should we understand it?