This is a markup issue in the Categories administration portlet.
Steps to reproduce:
1. Navigate Site Administration > Categories.
3. Add a new category.
4. Edit that category.
5. While in the Edit mode inspect source.
6. Search for "assetCategoryPanelContainer" id in the markup.
You should see that id used twice on the same page.
This issue can be also reproduced as followed:
1. From Categories use Add Vocabulary
2. Edit that vocabulary
3. Inspect source from the popup window
4. Search for "vocabularyExtraFieldsPanelContainer" id
There should only be 3 of them (because of the 'data-persist-id'), the other 3 came from the hidden instance