- for some reason we either (a) get duplicate events for user join from FS or (b) FS-ESL isn't handling
   the event properly. This results in duplicate entries on the user's list when user calls in from the
   phone. Check if a voice user with the same id is already in the voice conference and ignore the second
   event.
Merge pull request #438 from bigbluebutton/fix-duplicate-user-entries

 - for some reason we either (a) get duplicate events for user join from...