diff --git a/packages/backend/src/modules/auth/auth.repository.ts b/packages/backend/src/modules/auth/auth.repository.ts
index 0262cf6..9c747bb 100644
--- a/packages/backend/src/modules/auth/auth.repository.ts
+++ b/packages/backend/src/modules/auth/auth.repository.ts
@@ -17,6 +17,8 @@ export interface RefreshTokensRepository {
   findByHash(tokenHash: string): Promise<RefreshTokenRow | undefined>;
   revokeById(id: string): Promise<void>;
   revokeFamily(familyId: string): Promise<void>;
+  /** True when the family still has a live (unrevoked, unexpired) token. */
+  hasActiveInFamily(familyId: string): Promise<boolean>;
   revokeAllForUser(userId: string): Promise<void>;
   listActiveForUser(userId: string): Promise<RefreshTokenRow[]>;
 }
@@ -50,6 +52,18 @@ export const createRefreshTokensRepository = (
       .execute();
   },
 
+  hasActiveInFamily: async (familyId) => {
+    const row = await db
+      .selectFrom('refresh_tokens')
+      .select('id')
+      .where('family_id', '=', familyId)
+      .where('revoked_at', 'is', null)
+      .where('expires_at', '>', new Date())
+      .limit(1)
+      .executeTakeFirst();
+    return row !== undefined;
+  },
+
   revokeFamily: async (familyId) => {
     await db
       .updateTable('refresh_tokens')
diff --git a/packages/backend/src/modules/auth/auth.service.ts b/packages/backend/src/modules/auth/auth.service.ts
index e2cf146..04a618d 100644
--- a/packages/backend/src/modules/auth/auth.service.ts
+++ b/packages/backend/src/modules/auth/auth.service.ts
@@ -135,8 +135,18 @@ export const createAuthService = (deps: AuthServiceDeps): AuthService => {
         throw new HttpError(401, 'invalid_token', 'Invalid refresh token');
       }
       if (row.revoked_at !== null) {
-        await tokens.revokeFamily(row.family_id);
-        throw new HttpError(401, 'token_reuse', 'Refresh token reuse detected');
+        // Grace window: the same cookie replayed moments after rotation is a
+        // parallel client (second tab, double-mounted bootstrap), not theft.
+        // Re-issue within the family instead of nuking it. The grace only
+        // applies while the family still has a live successor — a family
+        // killed by reuse-detection or logout stays dead, so genuine theft
+        // can't ride the window back in.
+        const graceMs = 30_000;
+        const withinGrace = Date.now() - row.revoked_at.getTime() <= graceMs;
+        if (!withinGrace || !(await tokens.hasActiveInFamily(row.family_id))) {
+          await tokens.revokeFamily(row.family_id);
+          throw new HttpError(401, 'token_reuse', 'Refresh token reuse detected');
+        }
       }
       if (row.expires_at.getTime() <= Date.now()) {
         throw new HttpError(401, 'invalid_token', 'Refresh token expired');
@@ -145,7 +155,9 @@ export const createAuthService = (deps: AuthServiceDeps): AuthService => {
       if (user === undefined) {
         throw new HttpError(401, 'invalid_token', 'Invalid refresh token');
       }
-      await tokens.revokeById(row.id);
+      if (row.revoked_at === null) {
+        await tokens.revokeById(row.id);
+      }
       return issue(user, ctx, row.family_id);
     },
 
diff --git a/packages/core/src/api/auth.ts b/packages/core/src/api/auth.ts
index 9666325..18baaff 100644
--- a/packages/core/src/api/auth.ts
+++ b/packages/core/src/api/auth.ts
@@ -22,9 +22,22 @@ export const register = async (config: ApiClientConfig, body: RegisterBody): Pro
 export const login = async (config: ApiClientConfig, body: LoginBody): Promise<AuthResult> =>
   parse(authResultV, await requestJson(config, 'POST', '/auth/login', body));
 
-// Uses the httpOnly refresh cookie — no body.
-export const refresh = async (config: ApiClientConfig): Promise<AuthResult> =>
-  parse(authResultV, await requestJson(config, 'POST', '/auth/refresh'));
+// Uses the httpOnly refresh cookie — no body. Single-flighted: refresh rotates
+// the cookie, so two concurrent calls (double-mounted bootstrap effect, several
+// features racing on a 401) would replay the same token and trip the server's
+// reuse detection. All concurrent callers share one in-flight request.
+let inflightRefresh: Promise<AuthResult> | null = null;
+
+export const refresh = (config: ApiClientConfig): Promise<AuthResult> => {
+  inflightRefresh ??= (async () => {
+    try {
+      return parse(authResultV, await requestJson(config, 'POST', '/auth/refresh'));
+    } finally {
+      inflightRefresh = null;
+    }
+  })();
+  return inflightRefresh;
+};
 
 export const logout = async (config: ApiClientConfig): Promise<void> => {
   await requestJson(config, 'POST', '/auth/logout');