I've always found that the best way to profit from double-entry horses is to only look at the one's who are accepted in one race but an "emergency" in another. That shows that the horse is not in the 2nd race "just to have a run" as it ALREADY has one. If it switches to a better paying race due to scratchings....WATCH OUT.....at any price. Most obvious example in recent years....Dane Ripper:Cox Plate '97 at 40-1.
Also look at emergencies getting into a race where the trainer ALREADY has a more fancied horse going round,especially if the distance is right for ths emergency horse.
Good luck to all
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