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George A. Romero's Dawn of the Dead (1978) was released in Italy as Zombi. The following year, an Italian film with the working title of Gli Ultimi Zombi, was renamed Zombi 2. The title change was intended to cash in on Romero's movie, by presenting the film as an "unauthorized sequel" (or possibly prequel), although the plot bears little relation to the events portrayed in Dawn of the Dead.
See: this FAQ entry
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