diff --git a/api-specs/api/resources/customers.raml b/api-specs/api/resources/customers.raml
index eec264f90..f972f90b8 100644
--- a/api-specs/api/resources/customers.raml
+++ b/api-specs/api/resources/customers.raml
@@ -28,7 +28,8 @@ post:
   description: |
     If the `anonymousCart` field is set on the [CustomerDraft](ctp:api:type:CustomerDraft), then the newly created Customer will be assigned to that [Cart](ctp:api:type:Cart).
     Similarly, if the `anonymousId` field is set, the Customer will be set on all [Carts](ctp:api:type:Cart), [Orders](ctp:api:type:Order), [ShoppingLists](ctp:api:type:ShoppingList) and [Payments](ctp:api:type:Payment) with the same `anonymousId`.
-    Creating a Customer produces the [CustomerCreated](ctp:api:type:CustomerCreatedMessage) Message.
+
+    Creating a Customer produces the [CustomerCreated](ctp:api:type:CustomerCreatedMessage) Message. Simultaneously creating two Customers with the same email address can return a [LockedField](ctp:api:type:LockedFieldError) error.
   body:
     application/json:
       example: !include ../examples/customer-create.example.json
@@ -170,6 +171,7 @@ post:
     description: Checks if a Customer exists for a given `key`. Returns a `200 OK` status if the Customer exists or a `404 Not Found` otherwise.
   post:
     securedBy: [oauth_2_0: { scopes: ['manage_customers:{projectKey}'] }]
+    description: Simultaneously updating two Customers with the same email address can return a [LockedField](ctp:api:type:LockedFieldError) error.
     body:
       application/json:
         example: !include ../examples/customer-update.example.json
@@ -208,6 +210,7 @@ post:
     description: Checks if a Customer exists for a given `id`. Returns a `200 OK` status if the Customer exists or a `404 Not Found` otherwise.
   post:
     securedBy: [oauth_2_0: { scopes: ['manage_customers:{projectKey}'] }]
+    description: Simultaneously updating two Customers with the same email address can return a [LockedField](ctp:api:type:LockedFieldError) error.
     body:
       application/json:
         example: !include ../examples/customer-update.example.json
diff --git a/api-specs/api/resources/in-store.raml b/api-specs/api/resources/in-store.raml
index d010a6174..bee79c785 100644
--- a/api-specs/api/resources/in-store.raml
+++ b/api-specs/api/resources/in-store.raml
@@ -1458,7 +1458,8 @@ uriParameters:
       If the `anonymousCart` field is set on the [CustomerDraft](ctp:api:type:CustomerDraft), then the newly created Customer will be assigned to that [Cart](ctp:api:type:Cart).
       Similarly, if the `anonymousId` field is set, the Customer will be set on all [Carts](ctp:api:type:Cart), [Orders](ctp:api:type:Order), [ShoppingLists](ctp:api:type:ShoppingList) and [Payments](ctp:api:type:Payment) with the same `anonymousId`.
       If a Cart with a `store` field specified, the `store` field must reference the same [Store](ctp:api:type:Store) specified in the `{storeKey}` path parameter.
-      Creating a Customer produces the [CustomerCreated](ctp:api:type:CustomerCreatedMessage) Message.
+
+      Creating a Customer produces the [CustomerCreated](ctp:api:type:CustomerCreatedMessage) Message. Simultaneously creating two Customers with the same email address can return a [LockedField](ctp:api:type:LockedFieldError) error.
     body:
       application/json:
         example: !include ../examples/customer-create.example.json
@@ -1651,6 +1652,8 @@ uriParameters:
       displayName: Update customer by key in store
       description: |
         If the Customer exists in the Project but the `stores` field references a different Store, this method returns a [ResourceNotFound](ctp:api:type:ResourceNotFoundError) error.
+
+        Simultaneously updating two Customers with the same email address can return a [LockedField](ctp:api:type:LockedFieldError) error.
       securedBy:
         - oauth_2_0:
             scopes:
@@ -1714,6 +1717,8 @@ uriParameters:
       displayName: Update customer by id in store
       description: |
         If the Customer exists in the Project but the `stores` field references a different [Store](ctp:api:type:Store), this method returns a [ResourceNotFound](ctp:api:type:ResourceNotFoundError) error.
+
+        Simultaneously updating two Customers with the same email address can return a [LockedField](ctp:api:type:LockedFieldError) error.
       securedBy:
         - oauth_2_0:
             scopes:
diff --git a/api-specs/api/types/error/LockedFieldError.raml b/api-specs/api/types/error/LockedFieldError.raml
new file mode 100644
index 000000000..13e048f30
--- /dev/null
+++ b/api-specs/api/types/error/LockedFieldError.raml
@@ -0,0 +1,20 @@
+#%RAML 1.0 DataType
+(package): Error
+type: ErrorObject
+displayName: LockedFieldError
+discriminatorValue: LockedField
+description: |
+  Returned when two [Customers](ctp:api:type:Customer) are simultaneously created or updated with the same email address.
+
+  To confirm if the operation was successful, repeat the request.
+properties:
+  field:
+    type: string
+    description: |
+      Field that is currently locked.
+  code:
+    type: string
+  message:
+    type: string
+    description: |
+      `"'$field' is locked by another request. Please try again later."`
diff --git a/api-specs/api/types/error/graphql/GraphQLLockedFieldError.raml b/api-specs/api/types/error/graphql/GraphQLLockedFieldError.raml
new file mode 100644
index 000000000..4591d40e8
--- /dev/null
+++ b/api-specs/api/types/error/graphql/GraphQLLockedFieldError.raml
@@ -0,0 +1,16 @@
+#%RAML 1.0 DataType
+(package): Error
+type: GraphQLErrorObject
+displayName: GraphQLLockedFieldError
+discriminatorValue: LockedField
+description: |
+  Returned when two [Customers](ctp:api:type:Customer) are simultaneously created or updated with the same email address.
+
+  To confirm if the operation was successful, repeat the request.
+properties:
+  field:
+    type: string
+    description: |
+      Field that is currently locked.
+  code:
+    type: string
diff --git a/api-specs/api/types/types.raml b/api-specs/api/types/types.raml
index f981cd5e1..771c7e52e 100644
--- a/api-specs/api/types/types.raml
+++ b/api-specs/api/types/types.raml
@@ -525,6 +525,7 @@ InvalidOperationError: !include error/InvalidOperationError.raml
 InvalidSubjectError: !include error/InvalidSubjectError.raml
 InvalidTokenError: !include error/InvalidTokenError.raml
 LanguageUsedInStoresError: !include error/LanguageUsedInStoresError.raml
+LockedFieldError: !include error/LockedFieldError.raml
 MatchingPriceNotFoundError: !include error/MatchingPriceNotFoundError.raml
 MaxCartDiscountsReachedError: !include error/MaxCartDiscountsReachedError.raml
 MaxResourceLimitExceededError: !include error/MaxResourceLimitExceededError.raml
@@ -604,6 +605,7 @@ GraphQLInvalidOperationError: !include error/graphql/GraphQLInvalidOperationErro
 GraphQLInvalidSubjectError: !include error/graphql/GraphQLInvalidSubjectError.raml
 GraphQLInvalidTokenError: !include error/graphql/GraphQLInvalidTokenError.raml
 GraphQLLanguageUsedInStoresError: !include error/graphql/GraphQLLanguageUsedInStoresError.raml
+GraphQLLockedFieldError: !include error/graphql/GraphQLLockedFieldError.raml
 GraphQLMatchingPriceNotFoundError: !include error/graphql/GraphQLMatchingPriceNotFoundError.raml
 GraphQLMaxCartDiscountsReachedError: !include error/graphql/GraphQLMaxCartDiscountsReachedError.raml
 GraphQLMaxResourceLimitExceededError: !include error/graphql/GraphQLMaxResourceLimitExceededError.raml