Check that the email isn't used during the registration process

crates/handlers/src/views/register/password.rs

@@ -25,7 +25,7 @@ use mas_policy::Policy;
 use mas_router::UrlBuilder;
 use mas_storage::{
     queue::{QueueJobRepositoryExt as _, SendEmailAuthenticationCodeJob},
-    user::{UserEmailRepository, UserRepository},
+    user::{UserEmailFilter, UserEmailRepository, UserRepository},
     BoxClock, BoxRepository, BoxRng, RepositoryAccess,
 };
 use mas_templates::{
@@ -194,6 +194,13 @@ pub(crate) async fn post(
             state.add_error_on_field(RegisterFormField::Email, FieldError::Required);
         } else if Address::from_str(&form.email).is_err() {
             state.add_error_on_field(RegisterFormField::Email, FieldError::Invalid);
+        } else if repo
+            .user_email()
+            .count(UserEmailFilter::new().for_email(&form.email))
+            .await?
+            > 0
+        {
+            state.add_error_on_field(RegisterFormField::Email, FieldError::Exists);
         }
 
         if form.password.is_empty() {