docs: document registering user with existing email (#10859)

www/apps/resources/app/commerce-modules/auth/auth-flows/page.mdx

@@ -53,7 +53,7 @@ This method calls the `authenticate` method of the provider specified in the fir
 The basic authentication flow requires first using the `register` method, then the `authenticate` method:
 
 ```ts
-const { success, authIdentity } = await authModuleService.register(
+const { success, authIdentity, error } = await authModuleService.register(
   "emailpass",
   // passed to auth provider
   {
@@ -61,6 +61,12 @@ const { success, authIdentity } = await authModuleService.register(
   }
 )
 
+if (error) {
+  // registration failed
+  // TODO return an error
+  return
+}
+
 // later (can be another route for log-in)
 const { success, authIdentity, location } = await authModuleService.authenticate(
   "emailpass",
@@ -87,6 +93,15 @@ Check out the [AuthIdentity](/references/auth/models/AuthIdentity) reference for
 
 ![Diagram showcasing the basic authentication flow](https://res.cloudinary.com/dza7lstvk/image/upload/v1711373749/Medusa%20Resources/basic-auth_lgpqsj.jpg)
 
+### Auth Identity with Same Identifier
+
+If an auth identity, such as a `customer`, tries to register with an email of another auth identity, the `register` method returns an error. This can happen either if another customer is using the same email, or an admin user has the same email.
+
+There are two ways to handle this:
+
+- Consider the customer authenticated if the `authenticate` method validates that the email and password are correct. This allows admin users, for example, to authenticate as customers.
+- Return an error message to the customer, informing them that the email is already in use.
+
 ---
 
 ## Auth Flow 2: Third-Party Service Authentication