(Plug.Conn.AlreadySentError) the response was already sent

Using Guardian for register/login.
I am getting this error when I try to log in:

[info] Sent 401 in 819ms
[error] #PID<0.440.0> running WebPageWeb.Endpoint (cowboy_protocol) terminated
Server: localhost:4000 (http)
Request: POST /users/login_auth
** (exit) an exception was raised:
    ** (Plug.Conn.AlreadySentError) the response was already sent
        (plug) lib/plug/conn.ex:668: Plug.Conn.put_resp_header/3
        (phoenix) lib/phoenix/controller.ex:402: Phoenix.Controller.redirect/2
        (web_page) lib/web_page_web/controllers/user_controller.ex:1: WebPageWeb.UserController.action/2
        (web_page) lib/web_page_web/controllers/user_controller.ex:1: WebPageWeb.UserController.phoenix_controller_pipeline/2
        (web_page) lib/web_page_web/endpoint.ex:1: WebPageWeb.Endpoint.instrument/4
        (phoenix) lib/phoenix/router.ex:278: Phoenix.Router.__call__/1
        (web_page) lib/web_page_web/endpoint.ex:1: WebPageWeb.Endpoint.plug_builder_call/2
        (web_page) lib/plug/debugger.ex:122: WebPageWeb.Endpoint."call (overridable 3)"/2
        (web_page) lib/web_page_web/endpoint.ex:1: WebPageWeb.Endpoint.call/2
        (plug) lib/plug/adapters/cowboy/handler.ex:16: Plug.Adapters.Cowboy.Handler.upgrade/4
        (cowboy) src/cowboy_protocol.erl:442: :cowboy_protocol.execute/4

Controller:

def login(conn, _params) do
    changeset = Users.change_user(%User{})
    render(conn, "login.html", changeset: changeset)
  end

  def login_auth(conn, %{"user" => %{"email" => email, "password" => password}}) do
    Auth.authenticate_user(email, password)
    |> login_reply(conn)
  end

  defp login_reply({:error, error}, conn) do
    conn
    |> put_flash(:error, error)
    |> redirect(to: "/")
  end

  defp login_reply({:ok, user}, conn) do
    conn
    |> put_flash(:success, "Welcome back!")
    |> Guardian.Plug.sign_in(user)
    |> redirect(to: "/")
  end

Auth.ex

def authenticate_user(email, plain_text_password) do
    query = from u in User, where: u.email == ^email
    Repo.one(query)
    |> check_password(plain_text_password)
  end
 
  defp check_password(nil, _), do: {:error, "Incorrect email or password"}
 
  defp check_password(user, plain_text_password) do
    case Bcrypt.checkpw(plain_text_password, user.password_hash) do
      true -> {:ok, user}
      false -> {:error, "Incorrect email or password"}
    end
  end

Router:

defmodule WebPageWeb.Router do
  use WebPageWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipeline :api do
    plug :accepts, ["json"]
  end

  pipeline :auth do
    plug WebPage.Pipeline
  end

  pipeline :ensure_auth do
    plug Guardian.Plug.EnsureAuthenticated
  end

  scope "/", WebPageWeb do
    pipe_through [:browser, :auth] # Use the default browser stack

    get "/", PageController, :index
    post("/login_auth", UserController, :login_auth)
    get("/login", UserController, :login)
    get("/logout", UserController, :logout)
    resources "/users", UserController

  end

  scope "/admin", WebPageWeb do
    pipe_through [:browser, :auth, :ensure_auth]
    post("/login_auth", UserController, :login_auth)
    get "/", PageController, :index
    get("/login", UserController, :login)
    get("/logout", UserController, :logout)
    resources "/users", UserController

  end

  # Other scopes may use custom stacks.
  # scope "/api", WebPageWeb do
  #   pipe_through :api
  # end
end

So I investigated the problem and it comes out the verifications are allright.
Parameters which I give are correct but I couldn’t figure out where the the problem comes from.

After the login it saves the user into the connection regardless the given error.