diff --git a/backend/app/models.py b/backend/app/models.py
index 1948adc..41b860b 100644
--- a/backend/app/models.py
+++ b/backend/app/models.py
@@ -128,6 +128,9 @@ class UserPrediction(Base):
     __tablename__ = "user_predictions"
     __table_args__ = (
         UniqueConstraint("match_id", "device_id", name="uq_match_device"),
+        # 같은 이메일은 같은 경기에 1픽만 (NULL 이메일은 다수 허용 — NULL 은 서로 구별).
+        # 신규 DB 에만 자동 적용. 기존 테이블은 앱 로직(이메일 우선 식별)으로 보장.
+        UniqueConstraint("match_id", "email", name="uq_match_email"),
     )
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
diff --git a/backend/app/routers/predictions.py b/backend/app/routers/predictions.py
index 2beabaf..155b754 100644
--- a/backend/app/routers/predictions.py
+++ b/backend/app/routers/predictions.py
@@ -2,7 +2,7 @@
 from __future__ import annotations
 
 from fastapi import APIRouter, Depends, HTTPException
-from sqlalchemy import select, update
+from sqlalchemy import func, select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
 
@@ -63,15 +63,28 @@ async def submit_prediction(
 
     # outcome 은 스코어에서 도출 (입력과 불일치해도 스코어 기준으로 정규화)
     outcome = outcome_of(body.scoreA, body.scoreB)
+    email = str(body.email).strip().lower() if body.email else None
 
-    existing = (
-        await db.execute(
-            select(UserPrediction).where(
-                UserPrediction.match_id == body.matchId,
-                UserPrediction.device_id == body.deviceId,
+    # 중복 식별: ① 이메일(있으면 1차 신원 — 다른 기기여도 동일인) ② 기기(deviceId)
+    existing = None
+    if email:
+        existing = (
+            await db.execute(
+                select(UserPrediction).where(
+                    UserPrediction.match_id == body.matchId,
+                    func.lower(UserPrediction.email) == email,
+                )
             )
-        )
-    ).scalars().first()
+        ).scalars().first()
+    if existing is None:
+        existing = (
+            await db.execute(
+                select(UserPrediction).where(
+                    UserPrediction.match_id == body.matchId,
+                    UserPrediction.device_id == body.deviceId,
+                )
+            )
+        ).scalars().first()
 
     if existing:
         # 마감 전 1회 수정: 분포 보정 (이전 outcome 제거, 새 outcome 추가)
@@ -79,8 +92,9 @@ async def submit_prediction(
         existing.outcome = outcome
         existing.score_a = body.scoreA
         existing.score_b = body.scoreB
-        if body.email:
-            existing.email = str(body.email)
+        existing.device_id = body.deviceId  # 최신 제출 기기로 갱신
+        if email:
+            existing.email = email
         existing.notify = body.notify
         pred = existing
     else:
@@ -91,7 +105,7 @@ async def submit_prediction(
             outcome=outcome,
             score_a=body.scoreA,
             score_b=body.scoreB,
-            email=str(body.email) if body.email else None,
+            email=email,
             notify=body.notify,
         )
         db.add(pred)