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Ich freue mich, dass du gekommen bist.
Ich freue mich darüber, dass du gekommen bist.
I've seen both forms used to mean "I'm happy that you've come". Is one of them wrong?
No, both are fine. The second form may sound a bit more emphasized. So - slightly exaggerated - the first may be uttered when you enter the room, whereas the second may be an answer to "Why do you keep grinning all evening?"
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