Only the first Image Style is displayed in the Image dialog drop-down if there are multiple image styles with the same Type

Steps to reproduce

1. Go to Administer Wiki > Look & Feel > Image Styles
2. Create a new Image Style (e.g. Identifier: 'Style1', Pretty Name: Pretty1, Type: thumbnail)
3. Create again a new Image Style (e.g. Identifier: 'Style2', Pretty Name: Pretty2, Type: thumbnail)
4. Edit a page with CKEditor
5. Click on 'Image' from toolbar
6. Select an image from Gallery tab
7. Click 'Select' button
8. Observe the list of Image Styles displayed in the drop-down

Expected results

In the Image Styles drop-down, both 'Pretty1' and 'Pretty2' styles are displayed along with the '—' default style.

Actual results

Only 'Pretty1' and the default style '—' are displayed.

In order for 'Pretty2' to be displayed as well, the style Type must be changed to something different than the first entry (e.g. 'Pretty1') has or the style must be forced as the default style.

Tested and reproduced also on XWiki 15.10.6, 14.10.21 and 16.1.0.