Any baserunners ahead of the batter are also entitled to advance two bases on a ground rule double, based on their positions when the ball was pitched. This sometimes has the effect of denying a team a run, since a runner starting from first base would frequently come around to score on a normal double to the outfield, but in this case must stop at (or go back to) third base. This also means, that if a man is on second base, and a ground rule double is hit, he advances to score.
Since you played baseball, they created rules. I realize Doubleday probably didn't tell you at the time.
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