They mean essentially the same thing. If there is a difference, the extra word in the first one gives more emphasis and makes it more formal and serious-sounding. For example: A: I've seen a ufo.
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Anonymous What's the difference between "I'm being serious" and "I'm serious" ?Thanks!I'm being serious focuses on a temporary situation, not a permanent state of mind. I am being serious in this particular matter at this particular time. However, "I'm serious" may substitute for this meaning.