diff --git a/src/apps/api/idempotency.py b/src/apps/api/idempotency.py
new file mode 100644
index 000000000..035f4f0c7
--- /dev/null
+++ b/src/apps/api/idempotency.py
@@ -0,0 +1,98 @@
+import hashlib
+import json
+import logging
+
+from django.core.serializers.json import DjangoJSONEncoder
+from django.db import IntegrityError, transaction
+from rest_framework import status as http
+from rest_framework.response import Response
+
+from competitions.models import IdempotencyRecord
+
+logger = logging.getLogger(__name__)
+
+ENDPOINT_SUBMISSION_CREATE = "submissions.create"
+
+
+def _fingerprint(data):
+    """Stable sha256 over the request body."""
+    try:
+        canonical = json.dumps(data, sort_keys=True, default=str)
+    except TypeError:
+        canonical = repr(data)
+    return hashlib.sha256(canonical.encode("utf-8")).hexdigest()
+
+
+def _json_safe(data):
+    """Round-trip through DjangoJSONEncoder so UUID/datetime/Decimal survive."""
+    return json.loads(json.dumps(data, cls=DjangoJSONEncoder))
+
+
+class IdempotentCreateMixin:
+    """Replay-safe POST keyed by the `Idempotency-Key` header.
+
+    Behaviour:
+      * No header                       -> falls back to legacy create() (compat).
+      * Header + first call             -> runs create(), stores response, returns it.
+      * Header + replay, same payload   -> returns stored response (same id).
+      * Header + replay, diff payload   -> 409 Conflict.
+      * Header + in-flight (race)       -> 409 Conflict ("request already in flight").
+
+    The owner of the key is `request.user`; an anonymous user gets the legacy
+    path because there is no stable identity to scope the key.
+    """
+    idempotency_endpoint = ENDPOINT_SUBMISSION_CREATE
+
+    def create(self, request, *args, **kwargs):
+        key = request.headers.get("Idempotency-Key")
+        if not key or not request.user.is_authenticated:
+            return super().create(request, *args, **kwargs)
+
+        fp = _fingerprint(request.data)
+        owner = request.user
+        endpoint = self.idempotency_endpoint
+
+        try:
+            with transaction.atomic():
+                rec = (
+                    IdempotencyRecord.objects
+                    .select_for_update()
+                    .filter(owner=owner, endpoint=endpoint, key=key)
+                    .first()
+                )
+                if rec is None:
+                    rec = IdempotencyRecord.objects.create(
+                        owner=owner,
+                        endpoint=endpoint,
+                        key=key,
+                        request_fingerprint=fp,
+                    )
+                    is_new = True
+                else:
+                    is_new = False
+        except IntegrityError:
+            rec = IdempotencyRecord.objects.get(owner=owner, endpoint=endpoint, key=key)
+            is_new = False
+
+        if not is_new:
+            if rec.request_fingerprint != fp:
+                return Response(
+                    {"detail": "Idempotency-Key reused with a different payload"},
+                    status=http.HTTP_409_CONFLICT,
+                )
+            if rec.response_status:
+                return Response(rec.response_body, status=rec.response_status)
+            return Response(
+                {"detail": "Request already in flight for this Idempotency-Key"},
+                status=http.HTTP_409_CONFLICT,
+            )
+
+        resp = super().create(request, *args, **kwargs)
+
+        body = resp.data if isinstance(resp.data, (dict, list)) else {}
+        rec.response_status = resp.status_code
+        rec.response_body = _json_safe(body)
+        if isinstance(resp.data, dict) and "id" in resp.data:
+            rec.submission_id = resp.data["id"]
+        rec.save(update_fields=["response_status", "response_body", "submission", "updated_when"])
+        return resp
diff --git a/src/apps/api/views/submissions.py b/src/apps/api/views/submissions.py
index bfd6f8889..a3b7bfb0f 100644
--- a/src/apps/api/views/submissions.py
+++ b/src/apps/api/views/submissions.py
@@ -19,14 +19,15 @@
 from api.pagination import DynamicChoicePagination
 from tasks.models import Task
 from api.serializers.submissions import SubmissionCreationSerializer, SubmissionSerializer, SubmissionFilesSerializer, SubmissionDetailSerializer
-from competitions.models import Submission, SubmissionDetails, Phase, CompetitionParticipant
+from api.idempotency import IdempotentCreateMixin, ENDPOINT_SUBMISSION_CREATE
+from competitions.models import Submission, SubmissionDetails, Phase, CompetitionParticipant, IdempotencyRecord
 from leaderboards.strategies import put_on_leaderboard_by_submission_rule
 from leaderboards.models import SubmissionScore, Column, Leaderboard
 import logging
 logger = logging.getLogger(__name__)
 
 
-class SubmissionViewSet(ModelViewSet):
+class SubmissionViewSet(IdempotentCreateMixin, ModelViewSet):
     queryset = Submission.objects.all().order_by('-pk')
     permission_classes = []
     filter_backends = (DjangoFilterBackend, SearchFilter)
@@ -198,6 +199,32 @@ def create(self, request, *args, **kwargs):
                 raise ValidationError('You do not have participant permissions for this group')
         return super(SubmissionViewSet, self).create(request, *args, **kwargs)
 
+    @action(detail=False, methods=['get'], url_path=r'receipt/(?P<key>[^/]+)')
+    def receipt(self, request, key=None):
+        """Pollable receipt for a previous POST keyed by Idempotency-Key.
+
+        Returns 404 if no record exists, 202 if the request is in flight,
+        200 with the original response body once the create has finalised.
+        """
+        if not request.user.is_authenticated:
+            raise PermissionDenied('Authentication required')
+        try:
+            rec = IdempotencyRecord.objects.get(
+                owner=request.user,
+                endpoint=ENDPOINT_SUBMISSION_CREATE,
+                key=key,
+            )
+        except IdempotencyRecord.DoesNotExist:
+            return Response({'state': 'unknown'}, status=status.HTTP_404_NOT_FOUND)
+        if not rec.response_status:
+            return Response({'state': 'pending'}, status=status.HTTP_202_ACCEPTED)
+        return Response({
+            'state': 'completed',
+            'status': rec.response_status,
+            'submission_id': rec.submission_id,
+            'body': rec.response_body,
+        })
+
     def destroy(self, request, *args, **kwargs):
         """
         - If user is neither owner nor admin, user cannot delete the submission
diff --git a/src/apps/competitions/migrations/0062_idempotencyrecord.py b/src/apps/competitions/migrations/0062_idempotencyrecord.py
new file mode 100644
index 000000000..0ffb7d209
--- /dev/null
+++ b/src/apps/competitions/migrations/0062_idempotencyrecord.py
@@ -0,0 +1,67 @@
+# Generated by Django 5.2.14 on 2026-06-17 08:41
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("competitions", "0061_competition_participant_groups"),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name="IdempotencyRecord",
+            fields=[
+                (
+                    "id",
+                    models.BigAutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                ("key", models.CharField(max_length=128)),
+                ("endpoint", models.CharField(max_length=64)),
+                ("request_fingerprint", models.CharField(max_length=64)),
+                ("response_status", models.PositiveSmallIntegerField(default=0)),
+                ("response_body", models.JSONField(blank=True, default=dict)),
+                ("created_when", models.DateTimeField(auto_now_add=True)),
+                ("updated_when", models.DateTimeField(auto_now=True)),
+                (
+                    "owner",
+                    models.ForeignKey(
+                        on_delete=django.db.models.deletion.CASCADE,
+                        to=settings.AUTH_USER_MODEL,
+                    ),
+                ),
+                (
+                    "submission",
+                    models.ForeignKey(
+                        blank=True,
+                        null=True,
+                        on_delete=django.db.models.deletion.SET_NULL,
+                        related_name="+",
+                        to="competitions.submission",
+                    ),
+                ),
+            ],
+            options={
+                "indexes": [
+                    models.Index(
+                        fields=["created_when"], name="competition_created_672c32_idx"
+                    )
+                ],
+                "constraints": [
+                    models.UniqueConstraint(
+                        fields=["owner", "endpoint", "key"],
+                        name="unique_idempotency_owner_endpoint_key",
+                    ),
+                ],
+            },
+        ),
+    ]
diff --git a/src/apps/competitions/models.py b/src/apps/competitions/models.py
index 87b0b5513..caec67707 100644
--- a/src/apps/competitions/models.py
+++ b/src/apps/competitions/models.py
@@ -492,6 +492,9 @@ class Submission(models.Model):
     ingestion_worker_hostname = models.CharField(max_length=255, blank=True, null=True)
     # Scoring hostname
     scoring_worker_hostname = models.CharField(max_length=255, blank=True, null=True)
+    # Incremented every time a worker claims this submission from the broker.
+    # Used to detect redeliveries (M6) and provide an audit trail.
+    worker_attempt_count = models.PositiveIntegerField(default=0)
     queue = models.ForeignKey('queues.Queue', on_delete=models.SET_NULL, null=True, blank=True,
                               related_name='submissions')
     is_migrated = models.BooleanField(default=False)
@@ -809,3 +812,37 @@ class CompetitionWhiteListEmail(models.Model):
 
     def __str__(self):
         return f"{self.email} - Competition: {self.competition.title}"
+
+
+class IdempotencyRecord(models.Model):
+    """Tracks one client-attempted POST keyed by (owner, endpoint, key).
+
+    Used to make submission creation replay-safe: a client retrying the same
+    request with the same Idempotency-Key header receives the original response
+    instead of producing a duplicate row.
+    """
+    PENDING = 0
+    key = models.CharField(max_length=128)
+    owner = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
+    endpoint = models.CharField(max_length=64)
+    request_fingerprint = models.CharField(max_length=64)
+    response_status = models.PositiveSmallIntegerField(default=PENDING)
+    response_body = models.JSONField(default=dict, blank=True)
+    submission = models.ForeignKey(
+        'Submission', null=True, blank=True,
+        on_delete=models.SET_NULL, related_name='+',
+    )
+    created_when = models.DateTimeField(auto_now_add=True)
+    updated_when = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(
+                fields=['owner', 'endpoint', 'key'],
+                name='unique_idempotency_owner_endpoint_key',
+            ),
+        ]
+        indexes = [models.Index(fields=['created_when'])]
+
+    def __str__(self):
+        return f"IdempotencyRecord({self.endpoint}, {self.key}, status={self.response_status})"