Add email re-validation handling with UI feedback and backend validation to prevent duplicate email verification attempts

backend/src/main/java/com/alttd/altitudeweb/controllers/forms/MailController.java

@@ -33,6 +33,14 @@ public class MailController implements MailApi {
     @RateLimit(limit = 5, timeValue = 1, timeUnit = TimeUnit.HOURS, key = "mailSubmit")
     public ResponseEntity<MailResponseDto> submitEmailForVerification(SubmitEmailDto submitEmailDto) {
         UUID uuid = AuthenticatedUuid.getAuthenticatedUserUuid();
+        boolean emailAlreadyVerified = mailVerificationService.listAll(uuid).stream()
+                                           .filter(EmailVerification::verified)
+                                           .map(EmailVerification::email)
+                                           .anyMatch(mail -> mail.equalsIgnoreCase(submitEmailDto.getEmail()));
+        if (emailAlreadyVerified) {
+            throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Email already verified for user");
+        }
+
         EmailVerification saved = mailVerificationService.submitEmail(uuid, submitEmailDto.getEmail());
         MailResponseDto response = new MailResponseDto()
                 .email(saved.email())