util: fix argsman dupe key error

fixes #22638

If we find a duplicate key and error, clear `values` before returning so that
WriteSettings will write an empty file, therefore clearing it.

This aligns with GUI behaviour added in 1ee6d0b.

Author: willcl-ark

src/test/settings_tests.cpp

@@ -80,14 +80,15 @@ BOOST_AUTO_TEST_CASE(ReadWrite)
     BOOST_CHECK(values.empty());
     BOOST_CHECK(errors.empty());
 
-    // Check duplicate keys not allowed
+    // Check duplicate keys not allowed and that values returns empty if a duplicate is found.
     WriteText(path, R"({
         "dupe": "string",
         "dupe": "dupe"
     })");
     BOOST_CHECK(!util::ReadSettings(path, values, errors));
     std::vector<std::string> dup_keys = {strprintf("Found duplicate key dupe in settings file %s", fs::PathToString(path))};
     BOOST_CHECK_EQUAL_COLLECTIONS(errors.begin(), errors.end(), dup_keys.begin(), dup_keys.end());
+    BOOST_CHECK(values.empty());
 
     // Check non-kv json files not allowed
     WriteText(path, R"("non-kv")");

src/util/settings.cpp

@@ -99,6 +99,8 @@ bool ReadSettings(const fs::path& path, std::map<std::string, SettingsValue>& va
         auto inserted = values.emplace(in_keys[i], in_values[i]);
         if (!inserted.second) {
             errors.emplace_back(strprintf("Found duplicate key %s in settings file %s", in_keys[i], fs::PathToString(path)));
+            values.clear();
+            break;
         }
     }
     return errors.empty();