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.

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();