Your magnet may need to be rotated about the spoke to which it is mounted. A magnet has two poles and if it is mounted so that both poles pass near the pickup, it may register twice. Rotating the magnet to move one pole farther from the pickup should fix the problem. You can use a compass or another small magnet to find where the poles are located. Then move the magnet so that one pole passes close to and the other far from the pickup.