Once you have a lure rod and two two of the same lure (available in chapter four after you've recruited Olive), you can duplicate it as many times as you want to. just equip the lure rod and a lure. Then press R2 so you will have Monica displayed instead of Max. Now select the same lure and equip it as many times as you want, it will seem nothing will have happened, but you will now have multiple lures equiped and still have the one in the menu. once you remove the lures you can have up to 999 lures (don't go over 999 or it will reset the number to whatever you had before you did it) then you can sell them all for a lot of money.