There are two Great Flood legends: one in the Bible involving Noah, and one in Sumerian mythology. Multiple civilizations and cultures have a flood legend, so does that add historical credence to the possibility that a flood did happen, or is it just plain old legend plagiarism?
The story of Utnapishtim is adapted from the story about Atrahasis, which in turn is probably adapted from the earlier Babylonian flood myths. The same goes for the story in the bible. So to answer your question: Yes, it's "legendary plagiarism".
As to your original question, as these Babylonian flood myths are at least one thousand years older than the Biblical one, it is pretty obvious who copied from whom.
There is no doubt that a flood did happen. Floods happen all the time. There is no single shred of evidence that a global or biblical flood ever happened, and there most assuredly has never been any big flood created by a god to kill all of mankind except a few, with these few surviving because they were forewarned and built a boat. That is a children's story created not to be believed as fact, but to convey a moral.