[INTERNAL] ensure a contact can not invited multiple times at once

Added logic to ensure that a contact cannot be invited multiple times.
This change should always work as the check in the CollaborationUtils
class was too lazy.

Change-Id: I44ea8236b2f3a5798c66edd72f5de886edbaa3fe
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1288
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>