Have you ever written a line and wondered if it's actually yours? Sometimes I'll be writing and a line I've written seems vaguely familiar, and I can't guess whether it's because I thought of it myself or because I've read it before in someone else's work.
I need a better memory =(
I don't tend to wonder if somebody else "wrote" (came up with) a line I myself wrote, though I wouldn't say it's impossible; maybe it happens and I'm of course just not aware of it!
I do, however, often forget having written something until I read it again. This happens a lot, for better or for worse. It's usually with funny lines, it seems. I'll be proofreading or browsing something I wrote a few months ago and then will just burst out laughing at the monitor. I by no means think I'm anything near witty. But there are so many goofy lines I just can't remember writing, it's weird. Maybe it's just me though, whenever I read them to my parents they just give me odd looks like, "Why is that funny?"

I usually can't even remember where or how I came up with such silly lines, so I guess most of them aren't planned in advance.
A recurring dream of mine is one where I find some old childhood art or writing and it's in my own hand but I have no recollection of writing or even thinking about it, and there's this eerie "multiple personality/dissociative identity"-type feeling involved. *cue weird music*
I, too, need a much better memory.