A dual printed circuit board assembly, a printed circuit board, and a modular printed circuit board are provided. The printed circuit board includes a plurality of first connection points. The modular printed circuit board includes a plurality of second connection points. The modular printed circuit board is adapted to be mounted on the printed circuit board and includes a sensing unit, a first detecting unit, and a first notifying unit. The sensing unit outputs a detecting voltage according to a contact state between the first connection points and the second connection points. The first detecting unit determines whether the first connection points are respectively connected to the corresponding second connection points according to the detecting voltage. When one of the first connection points is not connected to the corresponding one of the second connection points, the first detecting unit controls the first notifying unit to issue a notification.
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