'嗯' and '嗯嗯' has a subtle difference. '嗯' can be used by anyone who wants to say yes, or who is thinking of an answer. “明天一起去看电影好吗？” "嗯" “明天一起去看电影好吗？” “嗯......好吧” thinking but finally decided. "嗯嗯" is more widely used by young people. It has two meanings. First it means agree with alacrity. "明天一起去看电影好吗？" "嗯嗯" usually accompanied with nodding，非常想去。 Second it means I do agree, don't bother to ask me again. "明天一起去看电影好吗？" “嗯” “明天看完电影一起去吃饭好吗？” “嗯” "明天看完电影吃饭完一起去唱歌好吗？" “嗯嗯” 意思是明天你说了算，不用再征求我的意见了。

in my opinion, they are similar. if you say 嗯, it means yes or okay to express you agree with someone if you say 嗯嗯, it still means you agree with others however, it sounds more sincere.

http://tw.knowledge.yahoo.com/question/question?qid=1405102804416 嗯嗯 作為擬聲詞 是親子間的童稚用語 是 排遺 之意 媽咪 我要嗯嗯 = 媽媽 我要大便

嗯 is more a sound than a word. It is used a bit like huh? or huh! Or even yeah. You use it express surprise, doubt or agreement. 嗯嗯 is the same thing just used twice.

嗯嗯 = yes