diff --git a/src/corelib/time/qtimezone.cpp b/src/corelib/time/qtimezone.cpp
index 64501907a9b..a89fe191a65 100644
--- a/src/corelib/time/qtimezone.cpp
+++ b/src/corelib/time/qtimezone.cpp
@@ -1486,8 +1486,18 @@ bool QTimeZone::isTimeZoneIdAvailable(const QByteArray &ianaId)
         || global_tz->backend->isTimeZoneIdAvailable(ianaId);
 }
 
+[[maybe_unused]] static bool isUniqueSorted(const QList<QByteArray> &seq)
+{
+    // Since [..., b, a, ...] isn't unique-sorted if a <= b, at least the
+    // suggested implementations of is_sorted() and is_sorted_until() imply a
+    // non-unique sorted list will fail is_sorted() with <= comparison.
+    return std::is_sorted(seq.begin(), seq.end(), std::less_equal<QByteArray>());
+}
+
 static QList<QByteArray> set_union(const QList<QByteArray> &l1, const QList<QByteArray> &l2)
 {
+    Q_ASSERT(isUniqueSorted(l1));
+    Q_ASSERT(isUniqueSorted(l2));
     QList<QByteArray> result;
     result.reserve(l1.size() + l2.size());
     std::set_union(l1.begin(), l1.end(),
@@ -1511,6 +1521,7 @@ static QList<QByteArray> set_union(const QList<QByteArray> &l1, const QList<QByt
 QList<QByteArray> QTimeZone::availableTimeZoneIds()
 {
     // Backends MUST implement availableTimeZoneIds().
+    // The return from each backend MUST be sorted and unique.
     return set_union(QUtcTimeZonePrivate().availableTimeZoneIds(),
                      global_tz->backend->availableTimeZoneIds());
 }
diff --git a/src/corelib/time/qtimezoneprivate_android.cpp b/src/corelib/time/qtimezoneprivate_android.cpp
index ec0710b1e4e..838f97c746c 100644
--- a/src/corelib/time/qtimezoneprivate_android.cpp
+++ b/src/corelib/time/qtimezoneprivate_android.cpp
@@ -203,14 +203,19 @@ QList<QByteArray> QAndroidTimeZonePrivate::availableTimeZoneIds() const
 {
     using namespace QtJniTypes;
 
-    const QJniArray androidAvailableIdList = TimeZone::callStaticMethod<String[]>("getAvailableIDs");
+    const QJniArray androidAvailableIdList
+        = TimeZone::callStaticMethod<String[]>("getAvailableIDs");
+    // Does not document order of entries.
 
-    QList<QByteArray> availableTimeZoneIdList;
-    availableTimeZoneIdList.reserve(androidAvailableIdList.size());
+    QList<QByteArray> result;
+    result.reserve(androidAvailableIdList.size());
     for (const auto &id : androidAvailableIdList)
-        availableTimeZoneIdList.append(id.toString().toUtf8());
+        result.append(id.toString().toUtf8());
 
-    return availableTimeZoneIdList;
+    // Sort & uniquify (just to be sure; it appears to not need this, but we can't rely on that).
+    std::sort(result.begin(), result.end());
+    result.erase(std::unique(result.begin(), result.end()), result.end());
+    return result;
 }
 
 QT_END_NAMESPACE
diff --git a/src/corelib/time/qtimezoneprivate_icu.cpp b/src/corelib/time/qtimezoneprivate_icu.cpp
index f42ffe07678..30e6b17a7cb 100644
--- a/src/corelib/time/qtimezoneprivate_icu.cpp
+++ b/src/corelib/time/qtimezoneprivate_icu.cpp
@@ -408,9 +408,10 @@ QList<QByteArray> QIcuTimeZonePrivate::availableTimeZoneIds() const
 {
     UErrorCode status = U_ZERO_ERROR;
     UEnumeration *uenum = ucal_openTimeZones(&status);
+    // Does not document order of entries.
     QList<QByteArray> result;
     if (U_SUCCESS(status))
-        result = uenumToIdList(uenum);
+        result = uenumToIdList(uenum); // Ensures sorted, unique.
     uenum_close(uenum);
     return result;
 }