Is it plagiarism if you don’t know?

For about 14 years I’ve been working on and off on a novella that became a novel and is now back to being a novella again. I was determined to wrap it up or chuck it out this year, and have been putting in some serious polishing work. Knowing I’d have to write a sales pitch for it if I was going to try get it published, I started reading some popular books in the sub-genre and just this morning came across, in a book by a HUGELY famous author, a scene that was almost exactly the same as a scene in my story. It was so weird. The originals of both mine and his stories were written around the same time, and I’m pretty sure I didn’t even subliminally copy his (and unless he was hanging round my loungeroom secretly I know he didn’t copy mine), but there it was.

What do you do in that situation? I’ve just rewritten the scene so it’s a little less eerily similar. But did I need to?