Why is the N-number on the tail of aircraft shown again much smaller and above the main N-number? They are both the same group of letters and numbers.
The N-Number marking regulations are outlined in this AC for fixed wing aircraft:
... minimum height of 12 inches
(1) On both surfaces of a single vertical tail or on the outer surfaces of a multi-vertical tail, or
(2) On the fuselage surfaces, on both sides of the fuselage between the trailing edge of the wing and the leading edge of the horizontal stabilizer. If engine pods or other equipment are located in this area and are an integral part of the fuselage side surfaces, you may place the marks on those pods or equipment.
However for some time 2 inch markings were allowed:
On an aircraft displaying 2-inch marks before November 1, 1981, and on aircraft manufactured between November 2, 1981, and January 1, 1983, you may display those marks until the aircraft is repainted or the marks are repainted, restored, or changed. Once the aircraft is repainted or the marks are repainted, restored, or changed, the N-number is required to be 12 inches high.
Its possible the numbers you are seeing are the old ones and the new 12 inch numbers were applied directly beneath it to comply with the new marking regulations while the old ones were simply not removed.