UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.84 Safari/537.36

Example URL:
https://gwynjudd.github.io/simple/

Steps to reproduce the problem:
1. Go to the page listed (https://gwynjudd.github.io/simple/)
2. Click the button "one"
3. Click the button "two"
4. Click the button "three"
5. Click the button "back" - note that you will see a message box "back"
6. Click the button "back" again - you will see the same message as before

Click OK on the dialog, this time you will also see the message:

Do you want to leave this site?

Changes you made may not be saved.

7. Click OK or cancel on the dialog - either way the same dialog will appear again. You cannot get past this dialog unless you check the box "Prevent this page from creating additional dialogs"

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Note that the code for the "back" button is very simple:

history.back();
alert('back');

If the "alert" is removed, the problem does not occur.

If the "beforeunload" handler is removed, or you check the box "Prevent this page from creating additional dialogs", the page actually does reload.

What is the expected behavior?
The second time you hit the "back" button, it should display the "back" alert, and go back in history the same as the first time.

What went wrong?
The page reloaded (or attempted to do so), instead of going back in history.

Does it occur on multiple sites: Yes

Is it a problem with a plugin? No 

Did this work before? N/A 

Does this work in other browsers? No Android embedded webview on Android N

Chrome version: 51.0.2704.84  Channel: n/a
OS Version: 10.0
Flash Version: Shockwave Flash 22.0 r0

Does not occur in MacOS Safari version 9.1.1
Does not occur in Edge 25.10586.0.0
Does not occur in IE 11.306.10586.0