IIUC the documentation every messages are returned only once and then removed from the internal handle.

But curl_multi::is_finished consumes all the queue searching for a specific easy handle, discarding the messages it is not interested in.

auto m = curl::curl_multi{};

m.add(easy1);
m.add(easy2);

// do something

std::cout << m.is_finished(easy1) << std::endl;
std::cout << m.is_finished(easy2) << std::endl;

The first call to curl_multi::is_finished could discard a message for easy2.

What if we add an unordered_map[easy_handle->message] in the curl_multi class in order to store the messages returned by curl_multi_info_read (overwrite the old ones should be ok)?

According to this answer on stackoverflow in order to reuse an easy handle you have to remove it, change and then re-add to the multi instance; if so we can cleanup the messages map during the remove phase.