diff --git a/classes/Customer.php b/classes/Customer.php
index b9b4b042071dc..20310b8a6d978 100644
--- a/classes/Customer.php
+++ b/classes/Customer.php
@@ -1210,6 +1210,11 @@ public function transformToCustomer($idLang, $password = null)
             return false;
         }
 
+        // If a customer with the same email already exists, wrong call
+        if (Customer::customerExists($this->email)) {
+            return false;
+        }
+
         $this->is_guest = false;
 
         /*
diff --git a/controllers/front/GuestTrackingController.php b/controllers/front/GuestTrackingController.php
index 944ce5bc33d6d..b51e3e8046a25 100644
--- a/controllers/front/GuestTrackingController.php
+++ b/controllers/front/GuestTrackingController.php
@@ -113,6 +113,13 @@ public function postProcess()
                     [],
                     'Shop.Notifications.Error'
                 );
+            // Check if a different customer with the same email was not already created in a different window or through backoffice
+            } elseif (Customer::customerExists($customer->email)) {
+                $this->errors[] = $this->trans(
+                    'You can\'t transform your account into a customer account, because a registered customer with the same email already exists.',
+                    [],
+                    'Shop.Notifications.Error'
+                );
             // Attempt to convert the customer
             } elseif ($customer->transformToCustomer($this->context->language->id, $password)) {
                 $this->success[] = $this->trans(
diff --git a/controllers/front/OrderConfirmationController.php b/controllers/front/OrderConfirmationController.php
index 038de3b78b339..c12d09d3d20c0 100644
--- a/controllers/front/OrderConfirmationController.php
+++ b/controllers/front/OrderConfirmationController.php
@@ -181,12 +181,22 @@ public function postProcess()
                 return;
             }
 
+            // Prevent error
+            // A) either on page refresh
+            // B) if we already transformed him in other window or through backoffice
             if ($this->customer->is_guest == 0) {
                 $this->errors[] = $this->trans(
                     'A customer account has already been created from this guest account. Please sign in.',
                     [],
                     'Shop.Notifications.Error'
                 );
+            // Check if a different customer with the same email was not already created in a different window or through backoffice
+            } elseif (Customer::customerExists($this->customer->email)) {
+                $this->errors[] = $this->trans(
+                    'You can\'t transform your account into a customer account, because a registered customer with the same email already exists.',
+                    [],
+                    'Shop.Notifications.Error'
+                );
             // Attempt to convert the customer
             } elseif ($this->customer->transformToCustomer($this->context->language->id, $password)) {
                 $this->success[] = $this->trans(