There's a glitch that allows you to use duplicate copies of items purchased from the Adult Shop. This trick has been known about since shortly after the game's launch in 2016, however, the glitch was poorly understood and not documented well until 2023.
Credit goes to AbiaIsMyWaifu for documenting this glitch properly. Since they explained it so well, we'll just quote what they wrote:
"In 2022, a glitch was discovered that allows you to reload your game and obtain more copies of previously used items in the process.
When the game saves your inventory, it doesn't clear the data that was there, and only saves what items are in filled slots. This means that stale data is left in item slots that were filled in a previous save, and that data can be loaded.
The game autosaves every day at midnight, and autosaves upon loading a different file or going back to the main menu. So, if you want to abuse the glitch, make sure to manually save your game if you've never saved with any inventory slots filled. But, you don't have to manually save before loading/returning to menu, it'll do that for you.
Let's say it's day 1, and your inventory from left to right is: Sub Pillow, Condom, Coke. You've manually saved the game with this inventory.
Now, you throw the Condom away or give it to a girl. Your inventory is now: Sub Pillow, Coke. If you reload your game, your inventory will now be: Sub Pillow, Coke, Coke.
Now, you throw/use your Sub Pillow, followed by your two Cokes, with no saving in between. If you reload your game, your inventory will now be: Sub Pillow, Coke, Coke. It never saved the in-between where your first slot had a Coke in it.
So in this theoretical, if you're only using 1 girl, it would probably be best to give her the Sub Pillow and 1 Coke. It lasts 24 hours so an autosave will occur before the Coke runs out. At that point, the next time you reload your game, your inventory will be Coke, Coke, Coke."