Notebook events triggered twice

[DonJayamanne]

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: Latest/Insiders
• OS Version: N/A

Steps to Reproduce:

1. Create an extension and handle the onDidChangeNotebookDocument event
2. Open a notebook
3. Type one character into a cell and watch the event get triggered twice.

@rebornix Possibly related to 749615f
Not sure, haven't had the change to git bisect.