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My husband has two real estate sites.  My plan, when I made the new one was to take the old one down.  But for some reason it mysteriously ranks well for several terms and continues to bring us business. With that being said, if there is duplicate content across the two sites then one is going to suffer. If the old site is ranking well, it may be ok to keep it.  But, it will be a challenge to optimize both sites.  It's much better to work on one site.  In most cases, I would redirect the old to the new site.  The only reason I would personally keep the old one is if it was ranking well already for them.