Forget about copying between stories. Heck, I can use the same phrase or concept twice in the same paragraph!

Actually, though, it's easy to use the same concept in two different stories without remembering that you used that concept before - especially when you've been writing stories for ten years.

ML wave (who suddenly feels very old laugh )

She was in such a good mood she let all the pedestrians in the crosswalk get to safety before taking off again.
- CC Aiken, The Late Great Lois Lane