Datapack Noob needs help with Loot Tables/Unique Items

Hey guys! I am a datapack noobie and I'm seeking some wisdom from my fellow datapack enjoyers. Currently, I am making a datapack for a server with all of my friends. However, one of my ideas that I wanted to implement, seems impossible to my knowledge. It is single-copy items generating in loot tables. The type where it gets removed from the loot pool/loot table once it has been generated by a player. I've seen it in Minecraft from content creators but I don't know if it was a special mod with that, or if it was a data pack. I've tried searching up the solutions and I've been staring at my VS Code page for almost 2 hours trying to rack my brain around it. I would love any input.