Well Kami's Dragon Balls only allowed a person to be revived only once, and it could only be if they died within a year before the wish was made. Guru's Dragon Balls didn't have that issue but they only allowed the revival of one person at a time. Dende's Dragon Balls, however, was like a mix of the two. People could be revived more than once with Dende's Dragon Balls, though his set had a catch of its own.