diff --git a/src/Entity/Mail/Message.php b/src/Entity/Mail/Message.php
index d0f3c542..55a29df3 100644
--- a/src/Entity/Mail/Message.php
+++ b/src/Entity/Mail/Message.php
@@ -19,7 +19,7 @@ use Doctrine\ORM\Mapping as ORM;
 // batch handlers dedup in PHP before inserting, but that check is a read on
 // stale data — this is the guard that actually holds when batches overlap
 // across runs or retries. Provider id leads the column list so the indexes
-// also serve the id-only lookups (findOneBy(['gmailId'|'graphId'])).
+// also serve the provider-id lookups (graph by account, gmail by owning user).
 #[ORM\UniqueConstraint(name: 'uniq_message_gmail_id_account', columns: ['gmail_id', 'account_id'])]
 #[ORM\UniqueConstraint(name: 'uniq_message_graph_id_account', columns: ['graph_id', 'account_id'])]
 // IMAP UIDs are unique within a mailbox, so this is the guard for the sync
diff --git a/src/Infrastructure/Messaging/Handler/SyncGmailMessageBatchHandler.php b/src/Infrastructure/Messaging/Handler/SyncGmailMessageBatchHandler.php
index 003e3578..852d6940 100644
--- a/src/Infrastructure/Messaging/Handler/SyncGmailMessageBatchHandler.php
+++ b/src/Infrastructure/Messaging/Handler/SyncGmailMessageBatchHandler.php
@@ -183,7 +183,7 @@ final readonly class SyncGmailMessageBatchHandler
             // redelivered batch — a worker restart, a retry after a partial
             // failure — used to build a second row for every message in it.
             if ('' !== $gmailId) {
-                $known = $this->messageRepository->findOneBy(['gmailId' => $gmailId]);
+                $known = $this->messageRepository->findOneByGmailIdForUser($account->usr, $gmailId);
 
                 if (null !== $known) {
                     // A native Gmail row: Gmail is the authority on its flags,
diff --git a/src/Repository/Mail/MessageRepository.php b/src/Repository/Mail/MessageRepository.php
index a9133088..0f3b74ac 100644
--- a/src/Repository/Mail/MessageRepository.php
+++ b/src/Repository/Mail/MessageRepository.php
@@ -1537,6 +1537,30 @@ class MessageRepository extends ServiceEntityRepository
             ->getResult();
     }
 
+    /**
+     * The one row for a Gmail id among the owner's accounts, for the batch
+     * handler's "already stored?" check.
+     *
+     * Scoped through the user and not the account, for the reason given on
+     * findByGmailIdsForUser(): a message fetched by one Gmail account may be
+     * attributed to a sibling. It used to be a bare findOneBy(['gmailId']),
+     * which searched every user on the install — a gmailId is only unique per
+     * account, so another user's row could be found and enriched with this
+     * user's labels. QueryBuilder because the owner is behind an association.
+     */
+    public function findOneByGmailIdForUser(User $user, string $gmailId): ?Message
+    {
+        return $this->createQueryBuilder('m')
+            ->innerJoin('m.account', 'a')
+            ->where('a.usr = :usr')
+            ->andWhere('m.gmailId = :gmailId')
+            ->setParameter('usr', $user)
+            ->setParameter('gmailId', $gmailId)
+            ->setMaxResults(1)
+            ->getQuery()
+            ->getOneOrNullResult();
+    }
+
     /**
      * QueryBuilder on two counts: the owner is reached through the account
      * association, which findBy() cannot traverse, and only one column comes
diff --git a/src/Service/Graph/GraphApiSyncer.php b/src/Service/Graph/GraphApiSyncer.php
index cddf53b6..cbd1c88e 100644
--- a/src/Service/Graph/GraphApiSyncer.php
+++ b/src/Service/Graph/GraphApiSyncer.php
@@ -217,7 +217,7 @@ final class GraphApiSyncer
             // Cast for the same reason, and here the symptom would be silent:
             // an int compared against a stored string id matches nothing, so
             // the message is simply not erased and nothing is raised.
-            $message = $this->messageRepository->findOneBy(['graphId' => (string) $graphId]);
+            $message = $this->messageRepository->findOneBy(['graphId' => (string) $graphId, 'account' => $account]);
 
             if (null === $message) {
                 continue;
@@ -294,7 +294,7 @@ final class GraphApiSyncer
             // phone stayed unread here, exactly as it did on IMAP, and for the
             // same reason — nothing ever re-read the state of a row it already
             // had.
-            $state = $this->flagStateOf($graphId, $item);
+            $state = $this->flagStateOf($account, $graphId, $item);
 
             if (null !== $state) {
                 $flagStates[] = $state;
@@ -323,7 +323,7 @@ final class GraphApiSyncer
      *
      * @param array<string,mixed> $item
      */
-    private function flagStateOf(string $graphId, array $item): ?RemoteFlagState
+    private function flagStateOf(Account $account, string $graphId, array $item): ?RemoteFlagState
     {
         if (false === array_key_exists('isRead', $item)) {
             // A delta entry that does not mention read state is not asserting
@@ -332,7 +332,7 @@ final class GraphApiSyncer
             return null;
         }
 
-        $message = $this->messageRepository->findOneBy(['graphId' => $graphId]);
+        $message = $this->messageRepository->findOneBy(['graphId' => $graphId, 'account' => $account]);
 
         if (null === $message) {
             return null;
@@ -355,7 +355,7 @@ final class GraphApiSyncer
 
     private function attachFolderLabel(Account $account, string $graphId, string $folderId): void
     {
-        $message = $this->messageRepository->findOneBy(['graphId' => $graphId]);
+        $message = $this->messageRepository->findOneBy(['graphId' => $graphId, 'account' => $account]);
 
         if (null === $message) {
             return;
@@ -404,7 +404,7 @@ final class GraphApiSyncer
 
     private function detachFolderLabel(Account $account, string $graphId, string $folderId): void
     {
-        $message = $this->messageRepository->findOneBy(['graphId' => $graphId]);
+        $message = $this->messageRepository->findOneBy(['graphId' => $graphId, 'account' => $account]);
 
         if (null === $message) {
             return;
diff --git a/tests/Service/Gmail/GmailInboundReadStateTest.php b/tests/Service/Gmail/GmailInboundReadStateTest.php
index f16196bb..35052eba 100644
--- a/tests/Service/Gmail/GmailInboundReadStateTest.php
+++ b/tests/Service/Gmail/GmailInboundReadStateTest.php
@@ -170,6 +170,31 @@ final class GmailInboundReadStateTest extends KernelTestCase
         self::assertNotNull($message->seenAt, 'the local change is still in flight');
     }
 
+    /**
+     * A gmailId is only unique per account. The "already stored?" lookup was
+     * a bare findOneBy(['gmailId']) across the whole install, so another
+     * user's row with the same id was found and rewritten with this user's
+     * labels, and this user's message was never stored at all.
+     */
+    public function testAnotherUsersRowWithTheSameGmailIdIsLeftAlone(): void
+    {
+        $mine          = $this->account;
+        $this->user    = $this->seedUser();
+        $this->account = $this->seedAccount();
+        $theirs        = $this->seedMessage(seen: false);
+        $this->account = $mine;
+
+        $this->handle(['INBOX']);
+
+        $this->em->refresh($theirs);
+
+        self::assertNull($theirs->seenAt, 'the other user\'s row must not be touched');
+        self::assertNotNull(
+            $this->em->getRepository(Message::class)->findOneBy(['gmailId' => self::GMAIL_ID, 'account' => $mine]),
+            'the message is stored for the account that fetched it',
+        );
+    }
+
     // ── helpers ───────────────────────────────────────────────────────────
 
     /**