One copy, two decks that both want it
You own one Rhystic Study. Your blue draw-go deck wants it. So does the Commander deck you built last month. Both decklists have it written down, and both of them are right — the rules never said a card could only appear on one list. What the rules cannot do is put the same piece of cardboard in two boxes at once.
That is the whole problem, and it is a physical one. It shows up on the cards you deliberately bought only once: Mana Vault, Smothering Tithe, The Great Henge, a dual land, the one expensive creature that three of your decks would happily run. Everything else you own in duplicate without thinking about it.
So the question is never "can I do this". It is "when I sit down to play, is this deck actually complete, or are four of its cards sitting in another box across the room?"
Option 1 — The staples box
Keep every shared card in one dedicated box, and build each deck around it. Before a game you pull what the deck needs; afterwards everything goes back. Nothing is ever "in" a deck, so nothing is ever missing from one.
This works, and it is the honest first answer for anyone with three or four decks. It costs you a few minutes of setup per game, and it fails in exactly one situation: the deck you have not touched in six months. You open the box, and you cannot tell whether it is ready to play or eight cards short.
Option 2 — Placeholder cards
Leave a slip of paper, a basic land or a printed placeholder in the sleeve where the borrowed card used to be. The deck stays at ninety nine cards, and the gap announces itself when you flip through.
It is cheap and it needs no tooling. It also only tells you that something is missing, never where it went — which is fine with two decks, and useless with seven. And the day you hand someone a deck to borrow, the placeholders are the first thing they will ask about.
Option 3 — Buy the second copy
Sometimes this is simply the right call. If two decks you play every week both want the same card, a duplicate ends the problem for good, and Commander is a format people play for years.
The trap is using it as a default. Buying a second copy of an expensive staple to avoid a two-minute swap is a very costly way to solve what is, underneath, a bookkeeping problem. Buy the duplicate when the schedule demands it, not when your memory does.
Option 4 — Keep a record that remembers for you
The fourth option is to stop relying on memory: write down which deck physically holds each shared card, and let the list tell you what is left to grab before you sit down. The decklists stay as they are — both decks still run the card — but exactly one of them holds it right now, and you can see which.
That is what Fetch does. A card declared in several decks stays declared in all of them; you tell Fetch which deck currently has the copy, and every other deck shows it as something to retrieve. Open a deck you have not played in months and the answer is immediate: five cards to collect, and here is where each one is.
Which one fits you
Two or three decks, played often, stored in one place: the staples box is enough, and anything else is overhead. Five decks and up, or decks that sleep for months between outings: you need something that survives your own forgetting, which means either duplicates for the handful of cards you cannot live without, or a written record.
These are not exclusive, and most players end up with a mix — a box for the shared cards, duplicates for the two or three that travel every week, and a list for everything else. What does not work is the fourth combination: no box, no duplicates, no list, and a deck you find out is incomplete once the game has already started.