Deduplicate data passed to FormPayload

lib/phoenix_test/form_payload.ex

@@ -3,12 +3,20 @@ defmodule PhoenixTest.FormPayload do
 
   def new(form_data) when is_list(form_data) do
     form_data
+    |> deduplicate_preserving_order()
     |> Enum.map_join("&", fn {key, value} ->
       "#{URI.encode_www_form(key)}=#{if(value, do: URI.encode_www_form(value))}"
     end)
     |> Plug.Conn.Query.decode()
   end
 
+  defp deduplicate_preserving_order(form_data) do
+    form_data
+    |> Enum.reverse()
+    |> Enum.uniq()
+    |> Enum.reverse()
+  end
+
   def add_form_data(payload, form_data) when is_map(payload) and is_list(form_data) do
     Enum.reduce(form_data, payload, fn {name, value}, acc ->
       with_placeholder = Plug.Conn.Query.decode("#{URI.encode_www_form(name)}=placeholder")

test/phoenix_test/form_payload_test.exs

@@ -2,6 +2,7 @@ defmodule PhoenixTest.FormPayloadTest do
   use ExUnit.Case, async: true
 
   alias PhoenixTest.Form
+  alias PhoenixTest.FormData
   alias PhoenixTest.FormPayload
 
   describe "new" do
@@ -107,6 +108,29 @@ defmodule PhoenixTest.FormPayloadTest do
       assert %{"hidden" => ["some_value"]} = FormPayload.new(form.form_data)
     end
 
+    test "deduplicates data with same name with [] and same value" do
+      form_data =
+        "email[]"
+        |> FormData.to_form_data("value")
+        |> FormData.add_data(FormData.to_form_data("email[]", "value"))
+
+      payload = FormPayload.new(form_data)
+
+      assert payload == %{"email" => ["value"]}
+    end
+
+    test "preserves order of operations when deduplicating data" do
+      form_data =
+        "email"
+        |> FormData.to_form_data("value")
+        |> FormData.add_data(FormData.to_form_data("email", "other_value"))
+        |> FormData.add_data(FormData.to_form_data("email", "value"))
+
+      payload = FormPayload.new(form_data)
+
+      assert payload == %{"email" => "value"}
+    end
+
     test "ignores hidden value for checkbox when checked" do
       html = """
       <form id="form">