[lang] Check for Duplicate Event Listener

[Matthew Aguirre]

I would like to add support to check for a duplicate event listener to the
org.apache.commons.lang3.event.EventListenerSupport class.
This would save me (and possibly others) from then checking the listeners
before adding.

   1. Should I just submit a ticket or should I implement a change within
   GitHub?
   2. Since I doubt that a global change would be appreciated, what would
   an acceptable method for signaling duplicate detection be?
      1. Flag in the constructor?
      2. Flag in the addListener method?

Thanks!
--
Matt