Identical treasure maps found in different shipwrecks.

I found #3 maps in three different shipwrecks, that they refer to the same treasure at the same "X" mark. It's actually three identical maps. Went and searched at the " X " mark, where the treasure is supposed to be hidden, found #1 treasure, made a search maybe there's another one, but I didn't find anything else. Why is that, is it a glitch? I'm scared that the next treasure map, will be the same treasure map as before and will take me to the same spot.