Content may be duplicated when the page is automatically saved in realtime

Steps to reproduce

1. Enable Realtime WYSIWYG from Administration
2. Login with an user (e.g. U1)
3. Edit a page (I reproduced by editing in-place, if relevant)
4. Using another (incognito) window/tab, login with another user (e.g. U2)
5. Edit the same page
6. Insert some text, then insert some images with each user
7. Add some other pieces of text with each user
8. Wait until the page gets automatically saved

Expected results

The content is saved properly one time, it's not duplicated.

Actual results

The content is saved, but it's almost fully duplicated, it seems only the first text line added on the page was not duplicated.

I've attached the screenshot and the XAR of the page where the issue was reproduced, but I couldn't successfully reproduce this issue consistently.

If I will find a way to reproduce this easier, I will update the steps.