Looking at other reports of this error message (here, here or here, for example), it seems like it most likely means that you've already used the name buttonSprite some other place in the script, perhaps in an import statement. Or you might have used it as the script name for some game entity (such as a GUI Control). Basically, it's telling you that the name is already in use.
There's also a mistake in line 73 in the code (going by the numbering in the forum post), since you're hardcoding it to use SaveGameSlot 1. You need to use lstSaveGamesList.SaveGameSlots[lstSaveGamesList.SelectedIndex] to show the currently selected savegame. (You may need to wrap it in a check so you don't get an "array index out of bounds" exception if nothing is selected in the listbox.)
Other than that, I find it a little weird that there's a bunch of code to ensure that if the player uses the same description as an existing savegame, it will overwrite it rather than save it as a separate savegame. That would really piss me off as a player.
There's also a mistake in line 73 in the code (going by the numbering in the forum post), since you're hardcoding it to use SaveGameSlot 1. You need to use lstSaveGamesList.SaveGameSlots[lstSaveGamesList.SelectedIndex] to show the currently selected savegame. (You may need to wrap it in a check so you don't get an "array index out of bounds" exception if nothing is selected in the listbox.)
Other than that, I find it a little weird that there's a bunch of code to ensure that if the player uses the same description as an existing savegame, it will overwrite it rather than save it as a separate savegame. That would really piss me off as a player.