How can the runtime efficiency of this function be improved?

I am trying to write optimal code (good runtime efficiency) for removing duplicate words in a sentence.

For example the input string Jack Juliet Juliet Jill Jack Romeo Jack Jill to the function should return Jack Juliet Jill Romeo

The following is my code :

std::string removeDuplicateWords(const std::string& str)
{
    std::stringstream ss (str);
    std::unordered_set<std::string> string_history;
    std::string current_string, output_string;
    ss >> current_string;
    string_history.insert(current_string);
    output_string += current_string;
    while(ss >> current_string) {
        if(string_history.find(current_string) == string_history.end()) {
            output_string += " " + current_string;
            string_history.insert(current_string);
        }
    }
    return output_string;
}