In my opinion, both are great, and it doesn't really matter which one you know because Americans and British people can understand each other just fine. There are some very subtle differences in spelling and vocabulary, but it never inhibits communication.
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February 11, 2017
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Answered
Both "I really don't know" and "I don't really know" are correct. "I don't know really" is incorrect.
Hope that helped! :)
