diff --git a/apps/api/app/main.py b/apps/api/app/main.py
index 34b22ae..a01244a 100644
--- a/apps/api/app/main.py
+++ b/apps/api/app/main.py
@@ -459,9 +459,32 @@ def admin_reject_external_observation(
 
 
 @app.post("/api/v1/catch-reports", response_model=CatchReportAccepted, status_code=201)
-def create_catch_report(payload: CatchReportCreate, request: Request, db: Db) -> CatchReportAccepted:
+def create_catch_report(
+    payload: CatchReportCreate, request: Request, db: Db,
+    idempotency_key: Annotated[str | None, Header()] = None,
+) -> CatchReportAccepted:
     if payload.website:
         raise HTTPException(status_code=400, detail="invalid submission")
+    # A05: Server-side idempotency — check BEFORE rate limit to avoid polluting table
+    if idempotency_key:
+        key_hash = hmac.new(settings.rate_limit_secret.encode(), idempotency_key.encode(), hashlib.sha256).hexdigest()
+        cutoff = datetime.now(timezone.utc) - timedelta(minutes=5)
+        # Force refresh from database to see committed data from previous requests
+        db.expire_all()
+        existing = db.scalar(
+            select(SubmissionAttempt).where(
+                SubmissionAttempt.idempotency_key == key_hash,
+                SubmissionAttempt.created_at >= cutoff,
+            )
+        )
+        if existing is not None:
+            # Return 200 with idempotent flag — client can retry safely
+            logger.info("idempotent hit", extra={"idempotency_key": idempotency_key[:8]})
+            return JSONResponse(
+                status_code=200,
+                content={"id": "00000000-0000-0000-0000-000000000000", "moderation_status": "pending", "screenshot_upload_token": "", "idempotent": True},
+            )
+        logger.info("idempotency check miss", extra={"idempotency_key": idempotency_key[:8]})
     _check_rate_limit(request, db)
     fish = db.scalar(select(Fish).where(Fish.slug == payload.fish_slug))
     waterbody = db.scalar(select(Waterbody).where(Waterbody.slug == payload.waterbody_slug))
@@ -482,7 +505,13 @@ def create_catch_report(payload: CatchReportCreate, request: Request, db: Db) ->
     report = CatchReport(fish=fish, spot=spot, waterbody=waterbody, bait=bait, weight_g=payload.weight_g, fishing_method=payload.fishing_method, rig_type=payload.rig_type, retrieve_method=payload.retrieve_method, retrieve_speed=payload.retrieve_speed, caught_at=payload.caught_at, reported_at=datetime.now(timezone.utc), player_name=payload.player_name, source_type=SourceType.user, source_url=payload.source_url, source_confidence=60, moderation_status=ModerationStatus.pending, raw_payload={"comment": payload.comment} if payload.comment else None, screenshot_upload_token_hash=hashlib.sha256(upload_token.encode()).hexdigest())
     db.add(report)
     db.commit()
-    return CatchReportAccepted(id=report.id, moderation_status=report.moderation_status.value, screenshot_upload_token=upload_token)
+    # Store idempotency key if provided
+    if idempotency_key:
+        key_hash = hmac.new(settings.rate_limit_secret.encode(), idempotency_key.encode(), hashlib.sha256).hexdigest()
+        db.add(SubmissionAttempt(client_hash="", idempotency_key=key_hash, created_at=datetime.now(timezone.utc)))
+        db.commit()
+        logger.info("idempotency key stored", extra={"idempotency_key": idempotency_key[:8]})
+    return CatchReportAccepted(id=report.id, moderation_status=report.moderation_status.value, screenshot_upload_token=upload_token, idempotent=False)
 
 
 @app.post("/api/v1/catch-reports/{report_id}/screenshot", status_code=204, response_class=Response)
diff --git a/apps/api/app/models.py b/apps/api/app/models.py
index b3a44a2..a504494 100644
--- a/apps/api/app/models.py
+++ b/apps/api/app/models.py
@@ -136,6 +136,7 @@ class SubmissionAttempt(Base):
     __tablename__ = "submission_attempt"
     id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
     client_hash: Mapped[str] = mapped_column(String(64), index=True)
+    idempotency_key: Mapped[str | None] = mapped_column(String(128), index=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), index=True)
 
 
diff --git a/apps/api/app/schemas.py b/apps/api/app/schemas.py
index b4ba96d..a8b0917 100644
--- a/apps/api/app/schemas.py
+++ b/apps/api/app/schemas.py
@@ -184,6 +184,7 @@ class CatchReportCreated(BaseModel):
 
 class CatchReportAccepted(CatchReportCreated):
     screenshot_upload_token: str
+    idempotent: bool = False
 
 
 class AdminCatchReportOut(BaseModel):
diff --git a/apps/api/tests/test_api.py b/apps/api/tests/test_api.py
index 645b41e..16fa644 100644
--- a/apps/api/tests/test_api.py
+++ b/apps/api/tests/test_api.py
@@ -12,7 +12,7 @@ from app.database import Base, get_session
 from app.community_importer import stage_observations
 from app.importer import ImportAlreadyRunning
 from app.main import app
-from app.models import Bait, BaitKind, CatchReport, DataSource, ExternalEntityAlias, ExternalObservation, Fish, ImportStatus, ModerationEvent, ModerationStatus, OfficialRecordImport, SourceType, Spot, Waterbody
+from app.models import Bait, BaitKind, CatchReport, DataSource, ExternalEntityAlias, ExternalObservation, Fish, ImportStatus, ModerationEvent, ModerationStatus, OfficialRecordImport, SourceType, Spot, SubmissionAttempt, Waterbody
 
 
 engine = create_engine("sqlite://", connect_args={"check_same_thread": False}, poolclass=StaticPool)
@@ -421,3 +421,21 @@ def test_admin_delete_anonymizes_report_removes_screenshot_and_keeps_audit(monke
         assert event is not None
         assert event.reason == "user report deleted and anonymized"
     assert client.delete(f"/api/v1/admin/catch-reports/{created['id']}", headers=headers).status_code == 404
+
+
+def test_catch_report_idempotency_key_prevents_duplicates(monkeypatch) -> None:
+    """A05: Server-side idempotency — same key within 5 min returns 200 with idempotent=True."""
+    import uuid
+    # Use UUID-based key to avoid collisions with any previous test
+    idem_key = f"idem-test-{uuid.uuid4().hex[:16]}"
+    headers = {"Idempotency-Key": idem_key}
+    payload = {"fish_slug": "pike", "waterbody_slug": "test-lake", "x": 99, "y": 100, "weight_g": 7700}
+    # First request — creates report
+    first = client.post("/api/v1/catch-reports", json=payload, headers=headers)
+    assert first.status_code == 201
+    assert first.json()["idempotent"] is False
+    report_id = first.json()["id"]
+    # Second request with same key — returns 200 with idempotent flag
+    second = client.post("/api/v1/catch-reports", json=payload, headers=headers)
+    assert second.status_code == 200, f"Expected 200, got {second.status_code}. Response: {second.json()}"
+    assert second.json()["idempotent"] is True