Answering before I read other answers: Assuming that the players had not interacted with the grave yard in a large way (Nadine trying to exile an already exiled deathrite, tarmogoyf counting creatures throughout the game when deathrite was the only creature, etc) then you'd want to use a GRV partial fix.

the violations that each player did for the first deathrite shaman (DRS) was Nadine having a clear GRV, and getting a warning, and Anna would be GRV also, so also getting a warning.

now the issue is the second DRS, because it was of the same nature and same misunderstanding/mistake that the first GRV, it has the same root cause, so each player should not get a second warning, but should be reminded to play more carefully.

Originally posted by Arthur Stifelman:

in case someone suspects Nadine acted in bad faith, an investigation should be started,

Thank you Arthur for giving me the opportunity to add a bit of clarification here, because I often see judges misinterpreting a such statement.

You don't start an investigation because you suspect something. You start an investigation when you're called to a table for anything else than a rule question. An investigation is what actually allow you to suspect something.

Whenever a judge answer a call, he or she should give a look to the game state, notice what are the life totals, cards in hand, whom is winning or losing, and what is the strategical impact of the mistake that has been committed. With habit, it takes about 30 seconds, and that's an investigation.
Only then, you know if something is suspicious, and if you need to press the investigation further, meaning immediately separating players and involving the Head Judge (if that's not yourself), or just apply the JAR or IPG and the recommended fix and penalty.

Also note that unless specified otherwise, Knowledge Pool scenarios always assume that you already investigated for cheating, and believe players committed honest mistakes. This is a shortcut that avoid every single answer to start with “investigate for cheating, disqualify if I believe someone is dishonest.”, which wouldn't add much to the discussion.

Time to wrap it up!

When each Deathrite Shaman was put into the graveyard during the resolution of Swords to Plowshares, the players violated the game rules. Specifically, an object changing zones moved to the wrong zone. And the controller of the Swords was not the player that carried out the zone change. The IPG specifically calls out actions to be taken under these circumstances.

Originally posted by IPG 2.5:

•If an object changing zones is put into the wrong zone, the identity of the object was known to all players, and it can be moved without disrupting the state of the game, put the object in the correct zone.

If a player takes an action called for by an effect controlled by his or her opponent, but does it incorrectly, both players receive a Game Play Error – Game Rule Violation. For example, if a player casts Path to Exile on an opponent’s creature and the opponent puts the creature into the graveyard, both players receive this infraction.

Each player receives a Warning for a Games Rules Violation. In addition, we will exile both Deathrite Shamans from the graveyard. The phrase “without disrupting the state of the game” refers to the current game state and should not take into account any intervening consequences of the error, such as Tarmogoyfs that had been given +1/+1 by the Deathrite during previous attacks. If cards that care about the contents of the graveyard are currently on the battlefield, some judgement may be required regarding whether to exile the Deathrite that died several turns before. However, in most situations there will not be an immediate, game-changing impact due to exiling the Deathrite and it should be moved to the correct zone.

Finally, we know that the error occurred twice. However, IPG 1.3. Applying Penalties tells us “if the root cause is the same, only the more severe one is applied.” In this case, the root cause is incorrectly resolving Swords to Plowshares. Since the errors were identical, neither is “more severe” and a single Warning is issued to each player.

Thanks to everyone for the great discussion, and we'll be back tomorrow with another scenario for your consideration!

I would like a small clarification around the IPG statement

“Separate infractions committed or discovered at the same time are treated as separate penalties, though if the root cause is the same, only the more severe one is applied. If the first penalty would cause the second one to be inapplicable for the round (such as a Game Loss issued along with a Match Loss), the more severe penalty is issued first, followed by the less severe penalty in the next round.”

Would the root cause be determined to be the same if the first Deathrite was killed by a Magma Spray instead of a Swords to Plowshares? Both spells say the Deathrite should be exiled instead of being placed in the graveyard.

Is the root cause considered to be the same if the Game Rule Violation is the same?

One minor point to add on that I haven't seen anyone else address:

Players don't always designate Graveyards and Exile zones in the same way. If both zones were empty at the time the first Path resolved, Anna would have no reason to assume that wherever the DRS was put was a Graveyard pile rather than an Exile pile. In that case, Anne's first infraction would be a FTMGS whenever Nadine (for example) cracked a fetchland and put it on top of Deathrite.

I've personally cast Last Breath on several early creatures and seen them go to places that typically are used for Graveyards. Often an opponent will put something into their Graveyard on the same pile (at which point I've corrected them). But several opponents have correctly started a second pile for their Graveyard without any necessary intervention.

One of the reasons we only give one penalty in these situations is that we give penalties mainly as a form of reinforcement of education. Giving away two penalties in this sort of case certainly wouldn't reinforce the players' education any more, so we don't give it. In situations where there are multiple types of mistakes made, it's worth running the situation through the head judge and letting them make a decision on which infraction(s) to give.

“Nadine gains 1 life, and Anna passes the turn. Nadine untaps and Anna sees that Nadine put the Shaman in her graveyard instead of exiling it.”

No significant actions has been taken between the bad resolution and the discovering of the error by Anna. Did she really did a GRV ? it looks like a rapid resolution of the turn like “StP on the Shaman, go” and she notices quite immediatly that Nadine put the shaman in the wrong zone.