Normally, duplicate questions are not deleted. The idea is that the duplicate helps finding the duplicated question.
Should duplicates asked from the same user in short time be voted to be deleted? (I am thinking of a period of time that is maximum an hour.) Does that depend from the text used for the question?
Should questions asked from a user who keeps creating a new question instead of editing the original one be voted to deletion too? Is this valid even if the questions are created every 8 hours?