diff --git a/aqt/archives.py b/aqt/archives.py
index 77eb5d76..214ca0e7 100644
--- a/aqt/archives.py
+++ b/aqt/archives.py
@@ -22,7 +22,7 @@
 import posixpath
 from dataclasses import dataclass, field
 from logging import getLogger
-from typing import Dict, Iterable, List, Optional, Tuple
+from typing import Dict, Iterable, List, Optional, Set, Tuple
 from xml.etree.ElementTree import Element  # noqa
 
 from defusedxml import ElementTree
@@ -293,7 +293,7 @@ def __init__(
         self.logger = getLogger("aqt.archives")
         self.archives: List[QtPackage] = []
         self.subarchives: Optional[Iterable[str]] = subarchives
-        self.mod_list: Iterable[str] = modules or []
+        self.mod_list: Set[str] = set(modules or [])
         self.is_include_base_package: bool = is_include_base_package
         self.timeout = timeout
         try:
diff --git a/tests/test_install.py b/tests/test_install.py
index fc373ff4..9cae23d4 100644
--- a/tests/test_install.py
+++ b/tests/test_install.py
@@ -575,6 +575,30 @@ def tool_archive(host: str, tool_name: str, variant: str, date: datetime = datet
                 r"INFO    : Time elapsed: .* second"
             ),
         ),
+        (  # Duplicates in the modules list
+            "install-qt windows desktop 6.1.0 win64_mingw81 -m qtcharts qtcharts qtcharts".split(),
+            "windows",
+            "desktop",
+            "6.1.0",
+            {"std": "win64_mingw81"},
+            {"std": "mingw81_64"},
+            {"std": "windows_x86/desktop/qt6_610/Updates.xml"},
+            {
+                "std": [
+                    plain_qtbase_archive("qt.qt6.610.win64_mingw81", "win64_mingw81"),
+                    qtcharts_module("6.1.0", "win64_mingw81"),
+                ]
+            },
+            re.compile(
+                r"^INFO    : aqtinstall\(aqt\) v.* on Python 3.*\n"
+                r"INFO    : Downloading qtbase...\n"
+                r"Finished installation of qtbase-windows-win64_mingw81.7z in .*\n"
+                r"INFO    : Downloading qtcharts...\n"
+                r"Finished installation of qtcharts-windows-win64_mingw81.7z in .*\n"
+                r"INFO    : Finished installation\n"
+                r"INFO    : Time elapsed: .* second"
+            ),
+        ),
         (
             "install-qt windows android 6.1.0 android_armv7".split(),
             "windows",