There's tunes, and there's a songs. I doubt if it's possible to make up a tune that isn't similar to some other tune. And if you could it'd sound horrible. (12 tone stuff and so forth, for example.)

Being similar doesn't mean being the same. It's the way someone will look like their parents. "He's got his father's nose" doesn't mean he stole it off him. Tunes belong to families as well as people

When you are putting a tune to a song, you want to have a tune that fits to the words, and fits to the the thought and sentiments and so forth. Whether it's original or not is very much a secondary consideration, all to do with money and that kind of rubbish.

Often if you're making a song, the words come easy, and you just write them down. It must sometimes come that people think they are writing a new song, and they are rewriting one that already exists.

I'm not sure it's ever happened to me. Here is one song which was so odd that I wondered whether it might have been a rewrite of one I'd heard or read somewhere - The Strange Conversation Maybe not as a sonmg, maybe as a poem or a story. If it rings a bell, let me know.