I've also heard that you're not supposed to freeze something more than once. Atleast thats what my gf keeps telling me. Keeps telling me I'll get sick from it.
Honestly if you think about it, by keeping food in the fridge after it's been in the freezer allows the bacteria/mold to grow because the temperature is closer to room temp(best temperature for bacteria/mold to grow). If you put it back into the freezer it should slow or stop the bacteria/mold from growing.
I think the reason they say that is it breaks down the food and ruins the taste. Same with letting it cool down to room temperature first. If you put it directly into the fridge or freezer it will cool down too quick and the taste won't be as good (true in some cases, but no big deal).