src/main/java/se/su/dsv/seshat/entities/AppUser.java

@@ -25,7 +25,7 @@ public class AppUser {
     @Column(nullable = false, unique = true)
     private String username;
 
-    @Column(nullable = false)
+    @Column(nullable = false, unique = true)
     private String email;
 
     @Column(nullable = false)

src/main/java/se/su/dsv/seshat/services/UserService.java

@@ -17,6 +17,9 @@ public class UserService {
         if (appUserRepository.existsByUsername(username)) {
             throw new IllegalArgumentException("Username already exists");
         }
+        if (appUserRepository.existsByEmail(email)) {
+            throw new IllegalArgumentException("Email already exists");
+        }
 
         AppUser newUser = new AppUser(username, email, "USER");
         appUserRepository.save(newUser);