If you select the git and the github plugins for install, the git plugin will be downloaded twice because the github plugin has a dependency on the git plugin.
(As shown in the office hours websession by rtyler )

Similar thing happens with instand messaging plugin. Quote from the chat:
"orrc: e.g. if you install "IRC" and "Jabber" plugin, it will install "Instant Messaging" twice, once for each"

Possible solution would be to check for duplicates before downloading plugins.