Welcome to the forum, Srikanth Dhondi.
We don't need to add "See you then". However, I wouldn't exactly call it redundant - it's part of the oil that lubricates conversation. Alternatively, one could say that 'sweet' is saying that 10 o'clock is a good time (the speaker had feared it might be at an earlier, less convenient time). The 'see you then' confirms that it is not just a good time, but that that the speaker will be at the meeting at that time.
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