There are variations of this trick, but mine is easier. The real trick is you have to have an even number of items, if there is an odd number then it won't work. And its a good idea to throw one of the duplicated items in the trash/fat chocobo--because if you -Sort- they revert back to an amount of 256--which the game reads as 0.
So, lets discuss how this happends and how to fix it?
Offtopic, and yes your ears aren't decieving you I remixed the overworld theme slightly.