Who are you talking about specifically? Some members had their user names changed (like Josie who switched from JosieGroper or Jow who switched from Jowseph) with admin consent. Others started new names (like mspaeth who stopped at 9999 posts and started anew at HudsonArcade, again with admin consent).
There have been banned members who've tried to come back under new names, and are usually banned again when they get caught.
There are others who have forgotten their passwords and just create a new name that is similar to their own (seen this recently with a couple new members). Eventually these duplicates will get purged from the system.
And then there are those who make a second name in order to perpetuate a joke (Kevin Sorbo).
If there is someone in particular you are referring to, we can see what category they fit...