added email_sent field to student model, do not send mail twice if not needed

cntmanage/flightslot/actions/send_email.py

@@ -31,6 +31,7 @@ def send_mail_password(request: HttpRequest, queryset: QuerySet[Student]) -> Non
         img.add_header("Content-Disposition", "inline", filename="cantorair.png")     
     
     # build mail list filling template
+    queryset = queryset.filter(mail_sent=False)
     mails: List[EmailMultiAlternatives] = []
     for student in queryset:
         if not student.user or not student.email: # skip student if has not an associated user
@@ -56,18 +57,25 @@ def send_mail_password(request: HttpRequest, queryset: QuerySet[Student]) -> Non
             mail.attach(filename=img)
             mail.attach_alternative(content=html_message, mimetype="text/html")
             mails.append(mail)
+            student.mail_sent = True
+            student.save()
         except Exception as e:
             messages.error(request=request, message=f"General Error: {e}")
     
+    if len(mails) == 0:
+        messages.warning(request=request, message="No email will be sent")
+        return 
+    
     # Open only one conenction and send mass email
     try:
+        sent: int = 0
         with get_connection() as conn:
-            conn.send_messages(mails)
+            sent = conn.send_messages(mails)
     except SMTPException as e:
         messages.error(request=request, message=f"Send Mail SMTP error: {e.strerror}")
     except Exception as e:
         messages.error(request=request, message=f"Send Mail General error: {e}")
     else:
-        messages.success(request=request, message=f"Successfully sent {len(mails)} messages")
+        messages.success(request=request, message=f"Successfully sent {sent} messages")
     
     return