Duplicated text after reopening the web app page

When closing the web app shortly after synchronisation and reopening it again, last entered text gets duplicated.

### Expected
There should be no duplicate words or lines.

### Observed
Words / characters become duplicated.
Once after deleting characters, double the amount of characters was deleted after reopening the page.

### Reproduced
I can reproduce the issue with a big success rate. Both on safari and chrome.

For example:
1. Go to https://app.simplenote.com
2. Pick a note
3. Add some characters to the note and wait for synchronisation to finish
4. Close the browser tab
5. Go to https://app.simplenote.com, characters are duplicated. There are 2 revisions added to the history, one with original characters and the final one with duplicated characters.


### Where did you see the bug

- System Make: Mac
- System Model:
- OS: macOS
- OS version: Catalina 10.15.7
- Browser safari (14) and chrome (Version 85.0.4183.121)
- Browser version (if applicable):
- Simplenote app version: app.simplenote.com

Reported on 3381437-zen

Reproduce the issue at https://app.simplenote.com using the listed synchronization, tab-closing, and reopening steps, checking the note history for the original and duplicated revisions. The payload names no source file or test, so trace the web app's editor and synchronization flow from that reproduction and verify that reopening preserves the text without duplicate characters or lines.

Written by the indexing model from the issue text.