More accordion items/entries at once open

[meventi-stephan]

In Bootstrap 3.2.0 it is at the moment possible to have more accordion items at once open. But normally only entry should be open.

How i made this:

1. Normal Click on accordion item "Collapsible Group Item #3"
2. Fast Click on accordion item "Collapsible Group Item #2"
3. Fast Click on accordion item "Collapsible Group Item #1" (as soon as possible after step 2))

Result (Image below -> two open accordion items):

[cvrebert]

Duplicate of #13447.