On rare occasion, an email can get lost and not find its recipient. Also, the teacher might have received it and meant to respond, but forgotten, in which case another email could be a suitable reminder.
But mainly it seems to be a carryover from social interaction in which you ask again if someone just sits there seeming to ignore your question.
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I'm using a non-verbal right now. I wish you could see it. --dyingofpoetry
NOT A DOCTOR