First, sorry for misunderstand your question. ;-;
I think there no way to prevent an overwriting from save your game process in a used save lot unless you would backup your own save files to other folder (which not located in your game folder) or use other save lots which available for you to save. (For me, I use so many save lots in order to try many possible was of choices)
FYI, the game will use save and persistent files from %appdata%/renpy/lustfuldesire... first. If there no save or persistent files in there, the game will use save and persistent files in your game folder. By the way, if you save your game process in a save slot which you already used, it will automatically overwrite the exist save files in both folders. That why I think you cannot prevent an overwriting from save your game process in a used save lot.
And one more thing, I'm sure about "share" that you talk about. Can you describe me more about this?
Sorry if it couldn't help.