diff --git a/api/app/Listeners/FormBuilder/TriggerPersonIdentityMatchOnFormSubmit.php b/api/app/Listeners/FormBuilder/TriggerPersonIdentityMatchOnFormSubmit.php
new file mode 100644
index 00000000..193ef4b9
--- /dev/null
+++ b/api/app/Listeners/FormBuilder/TriggerPersonIdentityMatchOnFormSubmit.php
@@ -0,0 +1,100 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Listeners\FormBuilder;
+
+use App\Enums\FormBuilder\FormPurpose;
+use App\Events\FormBuilder\FormSubmissionSubmitted;
+use App\Models\FormBuilder\FormSubmission;
+use App\Models\Person;
+use App\Services\PersonIdentityService;
+use Illuminate\Contracts\Queue\ShouldQueue;
+use Illuminate\Queue\InteractsWithQueue;
+use Illuminate\Support\Facades\Log;
+
+/**
+ * ARCH §31.1 — trigger PersonIdentityService::detectMatches on
+ * event_registration submissions and record the outcome on the
+ * submission so the portal can tell the submitter what's happening.
+ *
+ * States written to form_submissions.identity_match_status:
+ *   - 'matched' — the person is already linked to a user account
+ *   - 'pending' — one or more PersonIdentityMatch(pending) rows exist
+ *                 OR the submission is public (no subject yet; organiser
+ *                 will attach a person and matching runs later)
+ *   - 'none'    — the person exists, is unlinked, and nothing matched
+ *
+ * Failure mode per §31.1: log at error level, never rethrow so sibling
+ * listeners on the same event (§31.10 tag sync, §31.3 shift provisioning)
+ * keep running.
+ */
+final class TriggerPersonIdentityMatchOnFormSubmit implements ShouldQueue
+{
+    use InteractsWithQueue;
+
+    public string $queue = 'default';
+
+    public function __construct(
+        private readonly PersonIdentityService $identityService,
+    ) {}
+
+    public function handle(FormSubmissionSubmitted $event): void
+    {
+        try {
+            $submission = $event->submission->fresh(['schema']);
+            if ($submission === null) {
+                return;
+            }
+
+            $schema = $submission->schema;
+            if ($schema === null) {
+                return;
+            }
+
+            $purpose = $schema->purpose instanceof \BackedEnum
+                ? $schema->purpose->value
+                : (string) $schema->purpose;
+
+            if ($purpose !== FormPurpose::EVENT_REGISTRATION->value) {
+                return;
+            }
+
+            $status = $this->resolveStatus($submission);
+
+            // Use a raw UPDATE so we don't re-fire Eloquent events / an
+            // observer cascade on the submission itself.
+            FormSubmission::query()
+                ->whereKey($submission->id)
+                ->update(['identity_match_status' => $status]);
+        } catch (\Throwable $e) {
+            Log::error('form-builder.identity-match.listener_failed', [
+                'submission_id' => $event->submission->id,
+                'message' => $e->getMessage(),
+            ]);
+        }
+    }
+
+    private function resolveStatus(FormSubmission $submission): string
+    {
+        // Public submission without a person subject — mark pending so the
+        // portal shows the "we koppelen je gegevens zodra..." message.
+        // Real matching happens when the organiser attaches a person.
+        if ($submission->subject_type !== 'person' || $submission->subject_id === null) {
+            return 'pending';
+        }
+
+        $person = Person::withoutGlobalScopes()->find($submission->subject_id);
+        if ($person === null) {
+            return 'none';
+        }
+
+        if ($person->user_id !== null) {
+            return 'matched';
+        }
+
+        $matches = $this->identityService->detectMatches($person);
+
+        return $matches->isNotEmpty() ? 'pending' : 'none';
+    }
+}
diff --git a/api/app/Models/FormBuilder/FormSubmission.php b/api/app/Models/FormBuilder/FormSubmission.php
index b4c72099..12a88dca 100644
--- a/api/app/Models/FormBuilder/FormSubmission.php
+++ b/api/app/Models/FormBuilder/FormSubmission.php
@@ -50,6 +50,7 @@ final class FormSubmission extends Model
         'opened_at',
         'first_interacted_at',
         'idempotency_key',
+        'identity_match_status',
     ];
 
     /** @var array<string, string> */
diff --git a/api/app/Providers/AppServiceProvider.php b/api/app/Providers/AppServiceProvider.php
index bfa11ff2..8e54cdf8 100644
--- a/api/app/Providers/AppServiceProvider.php
+++ b/api/app/Providers/AppServiceProvider.php
@@ -46,6 +46,7 @@ use App\Models\FormBuilder\FormWebhookDelivery;
 use App\Models\VolunteerAvailability;
 use App\Events\FormBuilder\FormSubmissionSubmitted;
 use App\Listeners\FormBuilder\SyncTagPickerSelectionsOnSubmit;
+use App\Listeners\FormBuilder\TriggerPersonIdentityMatchOnFormSubmit;
 use App\Observers\FormBuilder\FormValueObserver;
 use App\Observers\PersonObserver;
 use App\Observers\UserObserver;
@@ -130,6 +131,12 @@ class AppServiceProvider extends ServiceProvider
             SyncTagPickerSelectionsOnSubmit::class,
         );
 
+        // ARCH §31.1 — identity-match trigger on event_registration.
+        \Illuminate\Support\Facades\Event::listen(
+            FormSubmissionSubmitted::class,
+            TriggerPersonIdentityMatchOnFormSubmit::class,
+        );
+
         ResetPassword::createUrlUsing(function ($user, string $token) {
             return config('crewli.portal_url') . '/wachtwoord-resetten?token=' . $token . '&email=' . urlencode($user->email);
         });
diff --git a/api/database/migrations/2026_04_22_100000_add_identity_match_status_to_form_submissions.php b/api/database/migrations/2026_04_22_100000_add_identity_match_status_to_form_submissions.php
new file mode 100644
index 00000000..db14a639
--- /dev/null
+++ b/api/database/migrations/2026_04_22_100000_add_identity_match_status_to_form_submissions.php
@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            // ARCH §31.1 + §31.10 integration signal. Populated by
+            // TriggerPersonIdentityMatchOnFormSubmit on the
+            // FormSubmissionSubmitted event; surfaced to the portal via
+            // PublicFormSubmissionResource.identity_match.
+            // Values: null | 'pending' | 'matched' | 'none'
+            $table->string('identity_match_status', 20)->nullable()->after('anonymised_at');
+            $table->index(
+                ['form_schema_id', 'identity_match_status'],
+                'fs_schema_identity_match_idx',
+            );
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            $table->dropIndex('fs_schema_identity_match_idx');
+            $table->dropColumn('identity_match_status');
+        });
+    }
+};
diff --git a/api/database/migrations/2026_04_22_100001_add_idempotency_key_unique_to_form_submissions.php b/api/database/migrations/2026_04_22_100001_add_idempotency_key_unique_to_form_submissions.php
new file mode 100644
index 00000000..4d783826
--- /dev/null
+++ b/api/database/migrations/2026_04_22_100001_add_idempotency_key_unique_to_form_submissions.php
@@ -0,0 +1,43 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        // S2c D4: a UNIQUE (form_schema_id, idempotency_key) is the race-
+        // safe backstop behind application-level idempotency replay. MySQL
+        // treats NULL as distinct in unique indexes, so rows without an
+        // idempotency_key remain unrestricted.
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            $table->dropIndex('fs_idempotency_idx');
+        });
+
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            $table->unique(
+                ['form_schema_id', 'idempotency_key'],
+                'fs_idempotency_unique',
+            );
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            $table->dropUnique('fs_idempotency_unique');
+        });
+
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            $table->index(
+                ['form_schema_id', 'idempotency_key'],
+                'fs_idempotency_idx',
+            );
+        });
+    }
+};