adding notes

This is a symptom of getting old! I always use this keystroke and now I've forgottten it and can't find it. I would highlight a note on a track. Then I would do a certain keystroke and it would make the note a hidden "double" so that if I raised or lowered the note, the original note stayed put and a new one would appear on the same stem and I could lower or raise it to a different pitch with up and down arrows and they would sould perfectly together in time.