Excel problem: How do I duplicate the contents of a field?

I have a lot of fields which will have duplicate contents, I enter contents into one field, then copy the content to another field (they’re edit documentation: “remove x from line y” “insert x to line z”, and I’m grading them for validity, on both pairs.
So, if I want to be lazy I put =yD into field zD, but that doesn’t really work, I need to lock down the contents so that zD doesn’t hold content “=yD” but rather replaces it with the result.
Is there a quick way to accomplish this?