diff --git a/src/main/java/it/cnr/isti/workflow/manager/auth/model/CreateUserResult.java b/src/main/java/it/cnr/isti/workflow/manager/auth/model/CreateUserResult.java
index a69b1e9..69fbf66 100644
--- a/src/main/java/it/cnr/isti/workflow/manager/auth/model/CreateUserResult.java
+++ b/src/main/java/it/cnr/isti/workflow/manager/auth/model/CreateUserResult.java
@@ -3,6 +3,7 @@ package it.cnr.isti.workflow.manager.auth.model;
 public enum CreateUserResult {
     SUCCESS,
     USER_ALREADY_EXISTS,
+    EMAIL_ALREADY_EXISTS,
     INVALID_EMAIL,
     INVALID_PASSWORD
 }
diff --git a/src/main/java/it/cnr/isti/workflow/manager/auth/repo/AuthRepository.java b/src/main/java/it/cnr/isti/workflow/manager/auth/repo/AuthRepository.java
index f655e70..31d0afc 100644
--- a/src/main/java/it/cnr/isti/workflow/manager/auth/repo/AuthRepository.java
+++ b/src/main/java/it/cnr/isti/workflow/manager/auth/repo/AuthRepository.java
@@ -28,4 +28,7 @@ public interface AuthRepository extends JpaRepository<LoginEntity, String> {
     @Query("select u from LoginEntity u where u.active is null or u.active = true")
     List<LoginEntity> findByActiveTrue();
 
+    @Query("select case when count(u) > 0 then true else false end from LoginEntity u where lower(u.email) = lower(:email)")
+    boolean existsByEmailIgnoreCase(@Param("email") String email);
+
 }
diff --git a/src/main/java/it/cnr/isti/workflow/manager/auth/repo/LoginEntity.java b/src/main/java/it/cnr/isti/workflow/manager/auth/repo/LoginEntity.java
index c2ff0dc..b3ea73e 100644
--- a/src/main/java/it/cnr/isti/workflow/manager/auth/repo/LoginEntity.java
+++ b/src/main/java/it/cnr/isti/workflow/manager/auth/repo/LoginEntity.java
@@ -1,6 +1,7 @@
 package it.cnr.isti.workflow.manager.auth.repo;
 
 import it.cnr.isti.workflow.manager.auth.model.UserRole;
+import jakarta.persistence.Column;
 import jakarta.persistence.Entity;
 import jakarta.persistence.EnumType;
 import jakarta.persistence.Enumerated;
@@ -17,6 +18,7 @@ public class LoginEntity {
     @Id
     private String username;
     private String password;
+    @Column(unique = true)
     private String email;
     private Boolean active;
     @Enumerated(EnumType.STRING)
diff --git a/src/main/java/it/cnr/isti/workflow/manager/auth/services/AuthService.java b/src/main/java/it/cnr/isti/workflow/manager/auth/services/AuthService.java
index c6730fa..82e97d7 100644
--- a/src/main/java/it/cnr/isti/workflow/manager/auth/services/AuthService.java
+++ b/src/main/java/it/cnr/isti/workflow/manager/auth/services/AuthService.java
@@ -49,14 +49,18 @@ public class AuthService {
         if (authRepository.existsById(username)) {
             return CreateUserResult.USER_ALREADY_EXISTS;
         }
-        if (!isValidEmail(email)) {
+        String normalizedEmail = normalizeEmail(email);
+        if (!isValidEmail(normalizedEmail)) {
             return CreateUserResult.INVALID_EMAIL;
         }
+        if (authRepository.existsByEmailIgnoreCase(normalizedEmail)) {
+            return CreateUserResult.EMAIL_ALREADY_EXISTS;
+        }
         if (!isValidPassword(password)) {
             return CreateUserResult.INVALID_PASSWORD;
         }
         String hashedPassword = hashPassword(password);
-        authRepository.save(new LoginEntity(username, hashedPassword, email.trim(), role == null ? UserRole.USER : role));
+        authRepository.save(new LoginEntity(username, hashedPassword, normalizedEmail, role == null ? UserRole.USER : role));
         return CreateUserResult.SUCCESS;
     }
 
@@ -164,7 +168,7 @@ public class AuthService {
         if (email == null) {
             return false;
         }
-        String normalizedEmail = email.trim();
+        String normalizedEmail = normalizeEmail(email);
         if (normalizedEmail.isEmpty() || normalizedEmail.contains(" ")) {
             return false;
         }
@@ -177,4 +181,8 @@ public class AuthService {
                 && dotIndex < normalizedEmail.length() - 1;
     }
 
+    private String normalizeEmail(String email) {
+        return email == null ? null : email.trim().toLowerCase(java.util.Locale.ROOT);
+    }
+
 }
diff --git a/src/main/java/it/cnr/isti/workflow/manager/controllers/AuthController.java b/src/main/java/it/cnr/isti/workflow/manager/controllers/AuthController.java
index 42e2ed0..b1418a4 100644
--- a/src/main/java/it/cnr/isti/workflow/manager/controllers/AuthController.java
+++ b/src/main/java/it/cnr/isti/workflow/manager/controllers/AuthController.java
@@ -90,7 +90,8 @@ public class AuthController {
         CreateUserResult result = authService.registerUser(request.getUsername(), request.getPassword(), request.getEmail());
         return switch (result) {
             case SUCCESS -> ResponseEntity.ok().build();
-            case USER_ALREADY_EXISTS -> ResponseEntity.badRequest().body("the user already exists");
+            case USER_ALREADY_EXISTS -> ResponseEntity.badRequest().body("USER_ALREADY_EXISTS");
+            case EMAIL_ALREADY_EXISTS -> ResponseEntity.badRequest().body("EMAIL_ALREADY_EXISTS");
             case INVALID_EMAIL -> ResponseEntity.badRequest().body("INVALID_EMAIL");
             case INVALID_PASSWORD -> ResponseEntity.badRequest().body("INVALID_PASSWORD");
         };
@@ -140,7 +141,8 @@ public class AuthController {
         CreateUserResult result = authService.createUser(request.username(), request.password(), request.email(), request.role());
         return switch (result) {
             case SUCCESS -> ResponseEntity.ok().build();
-            case USER_ALREADY_EXISTS -> ResponseEntity.badRequest().body("the user already exists");
+            case USER_ALREADY_EXISTS -> ResponseEntity.badRequest().body("USER_ALREADY_EXISTS");
+            case EMAIL_ALREADY_EXISTS -> ResponseEntity.badRequest().body("EMAIL_ALREADY_EXISTS");
             case INVALID_EMAIL -> ResponseEntity.badRequest().body("INVALID_EMAIL");
             case INVALID_PASSWORD -> ResponseEntity.badRequest().body("INVALID_PASSWORD");
         };
diff --git a/src/test/java/it/cnr/isti/workflow/manager/controllers/AuthControllerTest.java b/src/test/java/it/cnr/isti/workflow/manager/controllers/AuthControllerTest.java
index e980399..0db5611 100644
--- a/src/test/java/it/cnr/isti/workflow/manager/controllers/AuthControllerTest.java
+++ b/src/test/java/it/cnr/isti/workflow/manager/controllers/AuthControllerTest.java
@@ -303,6 +303,34 @@ public class AuthControllerTest {
         assert "INVALID_EMAIL".equals(response.getBody());
     }
 
+    @Test
+    public void registerRejectsDuplicateEmail() {
+        String email = "duplicate-" + uniqueSuffix() + "@example.com";
+
+        ResponseEntity<?> firstResponse = authController.register(
+                new AuthRequest("first-" + uniqueSuffix(), "Validpass1!", email));
+        assert firstResponse.getStatusCode().is2xxSuccessful();
+
+        ResponseEntity<?> secondResponse = authController.register(
+                new AuthRequest("second-" + uniqueSuffix(), "Validpass1!", email.toUpperCase(java.util.Locale.ROOT)));
+        assert secondResponse.getStatusCode() == HttpStatus.BAD_REQUEST;
+        assert "EMAIL_ALREADY_EXISTS".equals(secondResponse.getBody());
+    }
+
+    @Test
+    public void registerRejectsDuplicateUsername() {
+        String username = "duplicate-user-" + uniqueSuffix();
+
+        ResponseEntity<?> firstResponse = authController.register(
+                new AuthRequest(username, "Validpass1!", username + "@example.com"));
+        assert firstResponse.getStatusCode().is2xxSuccessful();
+
+        ResponseEntity<?> secondResponse = authController.register(
+                new AuthRequest(username, "Validpass1!", "other-" + uniqueSuffix() + "@example.com"));
+        assert secondResponse.getStatusCode() == HttpStatus.BAD_REQUEST;
+        assert "USER_ALREADY_EXISTS".equals(secondResponse.getBody());
+    }
+
     @Test
     public void adminEndpointsAcceptJwtAdminRole() throws Exception {
         String username = "jwtadmin-" + uniqueSuffix();