They mean the same thing, but the usage is different.
兄 is the older term and is somewhat old-fashioned. In modern mandarin it is typically not used by itself, but it still appears in combination with other characters in words like
兄弟姐妹 (siblings)
兄弟 (brother/brothers)
兄妹 (an older brother and younger sister)
老兄 (a familiar form of address between men)
Whereas 哥/哥哥 is often used by itself to directly address or refer to an older brother. It is also a component in other words like 表哥 and 哥们儿,but unlike 兄,哥/哥哥 is commonly used by itself.
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You can use 哥哥 as elder brother but you can’t use 兄 individually, although 兄means elder brother.
兄弟 means brothers in general .
Good friends can call each other 兄弟
你真是我的好兄弟。 you are my good brother (friend)
我们是兄弟。 we are brothers.
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兄弟 has two different meanings nowadays.
1. Elderly brother and younger brother. (亲兄弟means they are born from a same parent)
2.buddy / best friends as for guys.
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兄means elder brother弟means younger brother.
兄弟means elder brother and younger brother.
兄弟also has another meaning. Typically , It refers to an intimate friendship between men.
For example: 我们是好兄弟!
希望对您有帮助!
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兄 is the old way of saying older brother. In modern mandarin, it won't be used by itself, but it still be combined with other characters like 兄弟姐妹, which means siblings.
兄=Older Brother
弟= Younger Brother
姐= Older Sister
妹= Younger Sister