How can i keep Django summit form and show a message to user?

Question

I want to create a user when an article submit in Django admin form. I want to check a user is existed or not. After it, I want to show a message to user that user is existed, and he can change username to create.

Article model fields: family username

class WebsiteAdmin(admin.ModelAdmin):
        def save_model(self, request, obj, form, change):
            exist = User.objects.filter(username=obj.username).exists()
            if exist:
                messages.set_level(request, messages.ERROR)
                print('Username is already in use.')
            else:
                user = User.objects.create_user(obj.username, obj.email, obj.password,is_staff=True, is_active = True)
                user.first_name = obj.first_name
                user.last_name = obj.last_name
                user.save()   
                # messages.set_level(request, messages.ERROR)
                # messages.error(request, e)
                print('Username is not already in use.')
                super(WebsiteAdmin, self).save_model(request, obj, form, change)
    
    
    admin.site.register(Website,WebsiteAdmin)