A couple weeks ago I started fiddling with a riff during band practice and said to my brother "hey this sounds pretty good." He said "it should since it's that riff I came up with six months ago and taught you how to play!" Yeah, I thought this was coming to me a little too easily. Though at least some member of the band came up with it. He once started playing a Tom Cochrane song and was blown away as I appeared to come up with a lyrical melody on the spot. I was being cheeky.

My drummer tried to do that to me once but it doesn't work when you sing the lyrics in a different tune! He's remarkably bad at getting vocal melodies wrong. In karaoke it's like a mix of humour and frustration. "Dude you have this CD! How are you so unfamiliar with this song! You're not even singing in the same key! How are you, a songwriter and musician, fucking this up so badly?!"
What's funny about the "hey I wrote this song!" scenario I mentioned above is that when I hear the "real" song it sounds wrong to me. The chorus for "Do the Evolution" sounds to me like Eddie Vedder stealing the verse melody from one of my songs and trying to change it so it doesn't sound like a rip-off. But he's never heard my song. No song I've ever heard actually uses the vocal melody I came up with it. The awkward-to-me melody that Eddie Vedder is singing is what he came up with on his own. But it sounds fucking terrible to me.
"Everything's Gone Green" by New Order sounds weird to me because it sounds like an awkward re-write of "Blue Monday". But "Everything's Gone Green" was actually released FIRST, I just only heard "Blue Monday" first because it's the better-known song. So essentially New Order re-wrote one of their own songs and got something better.
I was very intolerant of this before I ever learned to play an instrument because I just assumed it was all intentional. But it really just happens. There are only 11 notes and just the written music we still have a record of goes back hundreds of years. Unless you're somehow inventing new notes anything you've written has been done before. It's really just how you arrange and present it.