Combo boxes don't display with duplicate id's

[guinness]

Steps to reproduce

1. Add two div sections to page with same id and contenteditable="true"
2. Click on 1st div and select styles dropdown - works
3. Click on 2nd div and select styles dropdown - fails

Expected result

Styles dropdown list should be shown on both

Actual result

Only first dropdown menu displays

Other details (browser, OS, CKEditor version, installed plugins)

Chrome/Edge, Windows 10, 4.5.5, Standard

Although this isn't a bug per se (duplicate id's), the editor should show a warning with an alert or console message to describe the problem as it's not obvious what the issue is.

[guinness]

Sample htm file reproducing problem

[Jakub Ś]

Problem can be reproduced from CKEditor 4.0.

IMHO CKEditor should inform that such instance already exists (just like in case of replace method) especially that CKEDITOR.instances.editor1 is created and available for usage.