diff --git a/services/nginx/app/modules/selfserve/traits/selfserve_lane_command_t.php b/services/nginx/app/modules/selfserve/traits/selfserve_lane_command_t.php
index 2efe709c..825f16ba 100644
--- a/services/nginx/app/modules/selfserve/traits/selfserve_lane_command_t.php
+++ b/services/nginx/app/modules/selfserve/traits/selfserve_lane_command_t.php
@@ -38,6 +38,49 @@ use objects\department_variables_o;
 
 trait selfserve_lane_command_t
 {
+
+    /**
+     * Acquire an atomic per-lane START lock before performing physical side effects.
+     */
+    protected function acquireLaneStartCommandLock(): string
+    {
+        if (!defined('redis') || !method_exists(redis, 'set_if_absent_with_expiration')) {
+            throw new \RuntimeException('Cannot start lane: START lock is unavailable.');
+        }
+
+        $token = bin2hex(random_bytes(16));
+        $lock_key = $this->getLaneStartCommandLockKey();
+        if (!redis->set_if_absent_with_expiration($lock_key, $token, 30)) {
+            throw new \RuntimeException("Cannot start lane: Lane is not available.");
+        }
+
+        return $token;
+    }
+
+    protected function releaseLaneStartCommandLock(string $token): void
+    {
+        if (!defined('redis')) {
+            return;
+        }
+
+        $lock_key = $this->getLaneStartCommandLockKey();
+        try {
+            if (method_exists(redis, 'get') && redis->get($lock_key) !== $token) {
+                return;
+            }
+            if (method_exists(redis, 'delete')) {
+                redis->delete($lock_key);
+            }
+        } catch (\Throwable) {
+            // The lock has a short TTL, so release failures must not mask START results.
+        }
+    }
+
+    protected function getLaneStartCommandLockKey(): string
+    {
+        return 'selfserve_lane_start_command_lock_' . (int)$this->id;
+    }
+
     /**
      * Determine if the lane and its department have self-serve enabled.
      * This method is intentionally protected to allow tests to override
@@ -509,38 +552,48 @@ trait selfserve_lane_command_t
                 // Validate customer number
                 if (!is_numeric($customer_number) || (int)$customer_number <= 0) throw new \InvalidArgumentException("Invalid customer number: " . $customer_number);
                 if (!(new users_o())->getUserByCustomerNumber((int)$customer_number)->exists()) throw new \InvalidArgumentException("Customer number does not exist: " . $customer_number);
-                $previous_customer_number = $this->getCustomerNumber();
-                $previous_license_plate = $this->getLicensePlate();
-                // Set the customer number and license plate
-                $this->setCustomerNumber($customer_number);
-                $this->setLicensePlate($license_plate);
+                $start_lock_token = $this->acquireLaneStartCommandLock();
                 try {
-                    // Open the entrance port before marking the lane occupied. Gateway timeouts are
-                    // ambiguous because the relay may already have received the pulse.
-                    $this->openEntrancePortForWashStart();
-                } catch (\Throwable $e) {
-                    $this->setCustomerNumber($previous_customer_number);
-                    $this->setLicensePlate($previous_license_plate);
-                    $this->setLaneState(selfserve_lane_state::IDLE);
-                    throw $e;
+                    // Re-check availability after taking the START lock so concurrent requests cannot
+                    // both pass the preflight check and trigger the physical entrance relay.
+                    if (!$this->getLaneStatus()->equals(selfserve_lane_status::AVAILABLE)) throw new \RuntimeException("Cannot start lane: Lane is not available.");
+                    $previous_customer_number = $this->getCustomerNumber();
+                    $previous_license_plate = $this->getLicensePlate();
+                    $previous_status = $this->getLaneStatus();
+                    // Set the customer number and license plate
+                    $this->setCustomerNumber($customer_number);
+                    $this->setLicensePlate($license_plate);
+                    // Mark the lane occupied before any physical entrance relay side effects.
+                    $this->setLaneStatus(selfserve_lane_status::OCCUPIED);
+                    try {
+                        // Gateway timeouts are ambiguous because the relay may already have received
+                        // the pulse, so openEntrancePortForWashStart() reports them and continues.
+                        $this->openEntrancePortForWashStart();
+                    } catch (\Throwable $e) {
+                        $this->setCustomerNumber($previous_customer_number);
+                        $this->setLicensePlate($previous_license_plate);
+                        $this->setLaneStatus($previous_status);
+                        $this->setLaneState(selfserve_lane_state::IDLE);
+                        throw $e;
+                    }
+                    // Set the lane state to IN_WASH
+                    $this->setLaneState(selfserve_lane_state::IN_WASH);
+                    // Start the wash timer
+                    $this->setWashStartTime(time());
+                    $this->runPublishedStudioActions(
+                        selfserve_studio_actions::EVENT_WASH_START_COMMAND,
+                        $this->resolveSelfServeActionWashModeForStart(),
+                        [
+                            'customer_number' => (int)$customer_number,
+                            'reg' => $license_plate,
+                        ]
+                    );
+                    $this->runRelaySideEffectsForWashStart($arguments);
+                    // Log the lane start event
+                    $this->logLaneAction(selfserve_lane_log_action::START_WASH);
+                } finally {
+                    $this->releaseLaneStartCommandLock($start_lock_token);
                 }
-                // Set the lane status to OCCUPIED when started
-                $this->setLaneStatus(selfserve_lane_status::OCCUPIED);
-                // Set the lane state to IN_WASH
-                $this->setLaneState(selfserve_lane_state::IN_WASH);
-                // Start the wash timer
-                $this->setWashStartTime(time());
-                $this->runPublishedStudioActions(
-                    selfserve_studio_actions::EVENT_WASH_START_COMMAND,
-                    $this->resolveSelfServeActionWashModeForStart(),
-                    [
-                        'customer_number' => (int)$customer_number,
-                        'reg' => $license_plate,
-                    ]
-                );
-                $this->runRelaySideEffectsForWashStart($arguments);
-                // Log the lane start event
-                $this->logLaneAction(selfserve_lane_log_action::START_WASH);
                 break;
             case selfserve_lane_command::STOP:
                 // Require lane to be occupied before stopping
diff --git a/services/nginx/app/tests/Unit/Selfserve/SelfserveLaneStartRaceLockWiringTest.php b/services/nginx/app/tests/Unit/Selfserve/SelfserveLaneStartRaceLockWiringTest.php
new file mode 100644
index 00000000..ee91df43
--- /dev/null
+++ b/services/nginx/app/tests/Unit/Selfserve/SelfserveLaneStartRaceLockWiringTest.php
@@ -0,0 +1,28 @@
+<?php
+
+it('locks and marks a self-serve start occupied before opening the entrance relay', function (): void {
+    $commandTrait = file_get_contents(app_path('modules/selfserve/traits/selfserve_lane_command_t.php'));
+
+    expect($commandTrait)->not->toBeFalse();
+    expect($commandTrait)->toContain('set_if_absent_with_expiration');
+
+    $startCaseOffset = strpos($commandTrait, 'case selfserve_lane_command::START:');
+    expect($startCaseOffset)->not->toBeFalse();
+
+    $startCase = substr($commandTrait, (int)$startCaseOffset, 3500);
+    $lockOffset = strpos($startCase, '$start_lock_token = $this->acquireLaneStartCommandLock();');
+    $occupiedOffset = strpos($startCase, '$this->setLaneStatus(selfserve_lane_status::OCCUPIED);');
+    $openOffset = strpos($startCase, '$this->openEntrancePortForWashStart();');
+    $finallyOffset = strpos($startCase, '} finally {');
+    $releaseOffset = strpos($startCase, '$this->releaseLaneStartCommandLock($start_lock_token);');
+
+    expect($lockOffset)->not->toBeFalse()
+        ->and($occupiedOffset)->not->toBeFalse()
+        ->and($openOffset)->not->toBeFalse()
+        ->and($finallyOffset)->not->toBeFalse()
+        ->and($releaseOffset)->not->toBeFalse()
+        ->and($lockOffset)->toBeLessThan($occupiedOffset)
+        ->and($occupiedOffset)->toBeLessThan($openOffset)
+        ->and($openOffset)->toBeLessThan($finallyOffset)
+        ->and($finallyOffset)->toBeLessThan($releaseOffset);
+});