system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some frozen products say "do not refreeze after defrosting"?

Is there something harmful about it?

If it was already frozen wouldn't it go back to the way it was?
gpt
It has to do with bacteria. When they initially froze it they froze it at the peak of freshness before a lot of bacteria was able to be introduced into it and multiply. When you thaw it out bacteria is able to grow and multiply and make it more of a health hazard. If you then refreeze it, you freeze all that bacteria along with it. So now, if you thaw it again you have an abundance of bacteria that is far more likely to cause food poisoning. 

Usually as long as you thouroughly cook something you really don't have a lot to worry about. However, not everyone follows proper cooking procedure so the companies add that to the box to cover themselves.