[BUG REPORT] Guests crash with assert when additions CD is inserted

[Yuri]

Host is amd64 10.1, guests I tried are several linuxes and windows. 
Insertion of additions CD worked the first time, when it was also 
downloaded. But the subsequent attempts all cause assertion, see log below.

Yuri


virtualbox-ose-4.3.20_4

--- error message ---
00:10:59.217392 !!Assertion Failed!!
00:10:59.217393 Expression: !m_mediums.contains(strMediumID)
00:10:59.217395 Location  : 
/usr/ports/emulators/virtualbox-ose/work/VirtualBox-4.3.20/src/VBox/Frontends/VirtualBox/src/medium/UIMediumEnumerator.cpp(118) 
void UIMediumEnumerator::createMedium(const UIMedium &)
/mnt/external/VirtualBox VMs/Arch/Logs/VBox.log 1166,1         Bot