Undo/redo is broken in some notebooks while editing cells

[AZHenley]

While editing a cell, if I undo or redo then the prior text is appended to the current text. This does not appear to happen in every notebook. It seems to only happen if you execute the cell between edits. (Chrome 89, Windows 10)

Example:

1. Go to the git example notebook.
2. Run the first cell.
3. Edit the cell to say dir.
4. Run the cell.
5. Click into the cell again and undo.

The content is now incorrect. If you redo then it will continue to append the text.

[ssmirr]

I wasn't able to reproduce this exact behavior with the steps above, however I see a similar thing (ctrl+z doesn't work) if you click outside the cells to unfocus, between each run. I will reply here when I find a fix for this.