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In ASA, the rule (2.3.H-1(c)) states that "on an errant throw pulling the defense into foul ground, the defense and the batter-runner may use either the white or the colored base."
That statement alone, IMHO, negates the entire usefulness of the double base. This scenario is usually the main reason for collisions at first base anyway and the double base does nothing to prevent this situation.
On a routine play at first, with well-coached players, a 15 inch should provide plenty of room for both players to do what they need to do without bodily contact. First basepersons should be coached to put their foot along the inside edge of the base, which at a minimum should leave at least 12 inches for the BR to plunk their size 15EEE (about 5 inches wide) on the base. It's the poorly coached players who stand with their foot right in the middle of the base that cause the problem, for those teams (and their opponents) the double base "crutch" will probably save a few ankles on the routine plays, but it still won't prevent the crashes caused by poor throws.
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SamC