The most common way of saying “You’re welcome,” is “どういたしまして.” This is a polite form that can be used with friends, colleagues, senior colleagues, supervisors and those significantly older than you. And you would use it in situations where you feel comfortable being thanked. In other words, if you thought your actions deserved an expression of appreciation and a mention. If someone thanks you for giving her a gift, or answering her question, you could say “どういたしまして,” because it’s pretty natural for you to be thanked for gestures like that. But if you think your actions did not deserve to be thanked or mentioned, you might want to say a phrase that means “Don’t mention it.” In Japanese, there is a casual and a formal way of saying this. In a casual situation, you would use “いいえ、いいえ” which is literally translated as “No, no” to mean “It’s no big deal.” For example, when someone thanks you for giving up your seat in the train or picking up something that they dropped, saying “いいえいいえ” conveys the feeling that you thought it was only natural for you to give up your seat (or pick up the object) and so you deserve no credit for it. But again, this would be used in response to casual gestures and situations. Most likely in such situations, they’ll thank you in a casual form as well, like “どうもありがとう.” Finally, If the person is older than the person who thanked him or her, the older person would respond "はい" meaning as "don't mention" or "it's okay". (Like a teenager thanked an elder lady. The elder lady would say "はい はい" in meaning of "don't mention" or "you are always welcome, kid".) If you are younger than the person who thank you, then はいはい would be an impolite response.

I agree with Brian's answer. Mind you, about 「いいえ、いいえ」 (iie, iie), more correctly term is 「いえ、いえ」 (ie, ie). This is, so to say, inflected forms. And when you still not fluency in Japanese, it's better to use 「どういたしまして」 or 「いえ、いえ」.

The best way to make it clear is to ask your friends directly why they answered in the way. I think you are allowed to ask because of a language learner. There's no strict rule that how we should answer actually. So whether a person says どういたしまして depends on his or her personality. I don't say どういたしまして so often, but sometimes maybe I say that. It really depends, in my case. I assume that just because you said ありがとうございます your friends didn't answer どういたしまして. But as you know ありがとうございます is more polite than ありがとう. So your friend's answer いいえいいえ、or どういたしまして seems more reasonable or like safer choice than to say どうも or はい when you say ありがとうございます. I sometimes say こちらこそありがとうございます instead of どういたしまして, depending on situations.