Have Polyphyodonts ever evolved from Diphyodonts?
Not true polydonty, True polydonty is an animals that grows endless numbers of teeth like reptiles and fish do.  Elephants, kangaroos, and manatees are sometimes called polydont but they are doing something very different, they still only grow a set number of teeth, but instead of growing them in two sets they spread them out growing them a few at a time. the total number of teeth is still the same as other mammals and once that last tooth falls out they can't make anymore. It is worth noting the teeth migrate forward in the jaw as they wear out unlike true polydont in which new teeth grow from below endlessly.In essence they are rationing that limited number of teeth. 

True polydonty has never evolved from a non-polydont ancestor.