diff --git a/lib/langue/formatter/json/parser.ex b/lib/langue/formatter/json/parser.ex
index a7d751d6..4e63f796 100644
--- a/lib/langue/formatter/json/parser.ex
+++ b/lib/langue/formatter/json/parser.ex
@@ -19,5 +19,6 @@ defmodule Langue.Formatter.Json.Parser do
     |> :jsone.decode(object_format: :tuple)
     |> elem(0)
     |> NestedParserHelper.parse()
+    |> Enum.uniq_by(& &1.key)
   end
 end
diff --git a/lib/langue/utils/parser.ex b/lib/langue/utils/parser.ex
index e65cb4de..0456a194 100644
--- a/lib/langue/utils/parser.ex
+++ b/lib/langue/utils/parser.ex
@@ -3,7 +3,5 @@ defmodule Langue.Formatter.Parser do
   alias Langue.Formatter.ParserResult, as: Output
   alias Langue.Formatter.SerializerResult, as: Input
 
-  @type result :: {:ok, Output.t()} | {:error, any()}
-
-  @callback parse(Input.t()) :: result
+  @callback parse(Input.t()) :: Output.t()
 end
diff --git a/lib/langue/utils/serializer.ex b/lib/langue/utils/serializer.ex
index 88a28530..4328d173 100644
--- a/lib/langue/utils/serializer.ex
+++ b/lib/langue/utils/serializer.ex
@@ -3,7 +3,5 @@ defmodule Langue.Formatter.Serializer do
   alias Langue.Formatter.ParserResult, as: Input
   alias Langue.Formatter.SerializerResult, as: Output
 
-  @type result :: {:ok, Output.t()} | {:error, any()}
-
-  @callback serialize(Input.t()) :: result
+  @callback serialize(Input.t()) :: Output.t()
 end
diff --git a/test/langue/json/exceptions_test.exs b/test/langue/json/exceptions_test.exs
index 2a18b200..23920be6 100644
--- a/test/langue/json/exceptions_test.exs
+++ b/test/langue/json/exceptions_test.exs
@@ -2,20 +2,28 @@ defmodule LangueTest.Formatter.Json.Exception do
   @moduledoc false
   use ExUnit.Case, async: true
 
+  alias Accent.FormatterTestHelper
   alias Langue.Formatter.Json
+  alias LangueTest.Formatter.Json.Expectation.DuplicateKey
   alias LangueTest.Formatter.Json.Expectation.InvalidFloatValue
   alias LangueTest.Formatter.Json.Expectation.InvalidIntegerValue
 
   Code.require_file("expectation_test.exs", __DIR__)
 
   test "invalid integer value" do
-    {expected_parse, result_parse} = Accent.FormatterTestHelper.test_serialize(InvalidIntegerValue, Json)
+    {expected_parse, result_parse} = FormatterTestHelper.test_serialize(InvalidIntegerValue, Json)
 
     assert expected_parse == result_parse
   end
 
   test "invalid float value" do
-    {expected_parse, result_parse} = Accent.FormatterTestHelper.test_serialize(InvalidFloatValue, Json)
+    {expected_parse, result_parse} = FormatterTestHelper.test_serialize(InvalidFloatValue, Json)
+
+    assert expected_parse == result_parse
+  end
+
+  test "duplicate key" do
+    {expected_parse, result_parse} = FormatterTestHelper.test_parse(DuplicateKey, Json)
 
     assert expected_parse == result_parse
   end
diff --git a/test/langue/json/expectation_test.exs b/test/langue/json/expectation_test.exs
index 5d039246..d2a4e3cf 100644
--- a/test/langue/json/expectation_test.exs
+++ b/test/langue/json/expectation_test.exs
@@ -126,6 +126,27 @@ defmodule LangueTest.Formatter.Json.Expectation do
     end
   end
 
+  defmodule DuplicateKey do
+    @moduledoc false
+    use Langue.Expectation.Case
+
+    def render do
+      """
+      {
+        "test": "foo",
+        "test": "bar",
+        "test": "baz"
+      }
+      """
+    end
+
+    def entries do
+      [
+        %Entry{index: 1, key: "test", value: "foo", value_type: "string"}
+      ]
+    end
+  end
+
   defmodule InvalidFloatValue do
     @moduledoc false
     use Langue.Expectation.Case