diff --git a/modeltrans/apps.py b/modeltrans/apps.py
index b187047..9e0dd6c 100644
--- a/modeltrans/apps.py
+++ b/modeltrans/apps.py
@@ -4,7 +4,7 @@
 import django.apps
 from django.apps import AppConfig
 
-from .conf import check_fallback_chain
+from .conf import check_fallback_chain, check_lang_code_duplicates
 from .translator import translate_model
 
 
@@ -14,6 +14,7 @@ class RegistrationConfig(AppConfig):
 
     def ready(self):
         check_fallback_chain()
+        check_lang_code_duplicates()
 
         for Model in django.apps.apps.get_models():
             translate_model(Model)
diff --git a/modeltrans/conf.py b/modeltrans/conf.py
index f043d2c..93d946e 100644
--- a/modeltrans/conf.py
+++ b/modeltrans/conf.py
@@ -26,12 +26,10 @@ def get_available_languages_setting():
     defaults to the list of language codes extracted from django setting LANGUAGES
     """
     languages = tuple(
-        set(
-            getattr(
-                settings,
-                "MODELTRANS_AVAILABLE_LANGUAGES",
-                (code for code, _ in getattr(settings, "LANGUAGES")),
-            )
+        getattr(
+            settings,
+            "MODELTRANS_AVAILABLE_LANGUAGES",
+            (code for code, _ in getattr(settings, "LANGUAGES")),
         )
     )
 
@@ -48,14 +46,13 @@ def get_available_languages(include_default=True):
     """
     Returns a tuple of available languages for django-modeltrans.
     """
-    MODELTRANS_AVAILABLE_LANGUAGES = get_available_languages_setting()
+    MODELTRANS_AVAILABLE_LANGUAGES = tuple(get_available_languages_setting())
 
     if include_default:
-        return tuple(
-            set(itertools.chain(MODELTRANS_AVAILABLE_LANGUAGES, (get_default_language(),)))
-        )
-    else:
-        return MODELTRANS_AVAILABLE_LANGUAGES
+        default_language = get_default_language()
+        if default_language not in MODELTRANS_AVAILABLE_LANGUAGES:
+            return tuple(itertools.chain((default_language,), MODELTRANS_AVAILABLE_LANGUAGES))
+    return MODELTRANS_AVAILABLE_LANGUAGES
 
 
 def check_fallback_chain():
@@ -77,6 +74,17 @@ def check_fallback_chain():
                 raise ImproperlyConfigured(message_fmt.format(l))
 
 
+def check_lang_code_duplicates():
+    available_languages = get_available_languages()
+    if len(available_languages) != len(set(available_languages)):
+        raise ImproperlyConfigured(
+            (
+                "MODELTRANS_AVAILABLE_LANGUAGES or LANGUAGES "
+                "should not contain duplicates, current list: {}"
+            ).format(available_languages)
+        )
+
+
 def get_fallback_chain(lang):
     """
     Returns the list of fallback languages for language `lang`.
diff --git a/tests/test_conf.py b/tests/test_conf.py
index bd06697..066b1c6 100644
--- a/tests/test_conf.py
+++ b/tests/test_conf.py
@@ -1,7 +1,12 @@
 from django.core.exceptions import ImproperlyConfigured
 from django.test import TestCase, override_settings
 
-from modeltrans.conf import check_fallback_chain, get_available_languages_setting
+from modeltrans.conf import (
+    check_fallback_chain,
+    check_lang_code_duplicates,
+    get_available_languages,
+    get_available_languages_setting,
+)
 
 
 class FallbackConfTest(TestCase):
@@ -37,3 +42,16 @@ def test_available_languages_should_be_str(self):
         message = "MODELTRANS_AVAILABLE_LANGUAGES should be an iterable of strings"
         with self.assertRaisesMessage(ImproperlyConfigured, message):
             get_available_languages_setting()
+
+    @override_settings(MODELTRANS_AVAILABLE_LANGUAGES=("nl", "nl", "de"))
+    def test_lang_code_duplicates_validation(self):
+        message = (
+            "MODELTRANS_AVAILABLE_LANGUAGES or LANGUAGES should not contain duplicates, current list: {}"
+        ).format(get_available_languages())
+        with self.assertRaisesMessage(ImproperlyConfigured, message):
+            check_lang_code_duplicates()
+
+    @override_settings(MODELTRANS_AVAILABLE_LANGUAGES=("nl", "bg", "ar"), LANGUAGE_CODE="en")
+    def test_languages_ordering(self):
+        self.assertEqual(("en", "nl", "bg", "ar"), get_available_languages())
+        self.assertEqual(("nl", "bg", "ar"), get_available_languages(include_default=False))
diff --git a/tox.ini b/tox.ini
index 3b23571..e6228cf 100644
--- a/tox.ini
+++ b/tox.ini
@@ -23,7 +23,7 @@ basepython =
 
 # workaround for Error installing '...django-tables2': editable mode is not supported for pyproject.toml-style projects.
 # https://github.com/pypa/pip/issues/6434
-install_command = python -m pip install --no-use-pep517 {opts} {packages}
+# install_command = python -m pip install --no-use-pep517 {opts} {packages}
 
 usedevelop = true
 pip_pre = true