How to prevent user to access login page in django when already logged in?

You can use this decorator as well.

def login_excluded(redirect_to):
    """ This decorator kicks authenticated users out of a view """ 
    def _method_wrapper(view_method):
        def _arguments_wrapper(request, *args, **kwargs):
            if request.user.is_authenticated:
                return redirect(redirect_to) 
            return view_method(request, *args, **kwargs)
        return _arguments_wrapper
    return _method_wrapper

Then call it in your views.py.

@login_excluded('app:redirect_to_view')
def someview(request):
    # ...

You can redirect users by modifying your urls.py file like below:

from django.urls import path
from django.contrib.auth import views as auth_views
from . import views

app_name = 'account'

urlpatterns = [
  path('signup/', views.register, name='register'),
  path('', auth_views.LoginView.as_view(redirect_authenticated_user=True), name='login'),
]

This will redirect already authenticated users from the login page. For the signup you will have to customize your register function add an if user is authenticated check.