Error when saving Charcoal color theme

1. Login as admin on a clean instance of XE 7.4.2 (with the default DB) and open http://localhost:8080/xwiki/bin/admin/XWiki/XWikiPreferences?editor=globaladmin&section=Presentation
2. By default, the color theme is Charcoal. Change the color theme.
3. Now, change the color theme back to Charcoal and double click on Save button.

Actual result:

• You get this error , displayed only if you have other color theme set and you change it into Charcoal: https://up1.xwikisas.com/#PgPaQnG4tQine4c2CZ3_8Q

  A problem occurred while trying to process your request. Please contact the webmaster if this happens again.
  
  Detailed information:
      Error number 3201 in 3: Exception while saving document xwiki:XWiki.XWikiPreferences
  com.xpn.xwiki.XWikiException: Error number 3201 in 3: Exception while saving document xwiki:XWiki.XWikiPreferences
  ...
  Caused by: org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session: [com.xpn.xwiki.doc.XWikiDocument#6152552094868048244]
  

Expected result:

• If I click once on Save button, I expect to be blocked the 2nd click, like in the case of Install button for extensions.