Let's examine the death of Bob Kirkland.  The reason that he asked Lennon if he recognized him before he killed him is that he wanted to know if his identical twin brother is still alive, and if Lennon had said yes, he would have asked him where he saw him, etc.  He also asked other suspects on the fake list if they recognized him, because if any of them were actually RJ, they would have known his appearance as Mark Kirkland.  However, since members of the Tyger-Tyger gang can lie so easily to nonmembers (Reede Smith to Bob Kirkland), that may not be an easy way to find RJ.  When Smith let Kirkland out of the back of the Police van, Kirkland knew his number was up, because he knew too much about the Tyger group, that is why he asked Smith 'Why not do it here' and Smith replied 'Protocol'.  The Tyger-Tyger group is obviously very wide spread, including police drivers in its' group.  It is going to be difficult for PJ, who doesn't know yet about the group.  Since the actor who plays Kirkland doesn't appear in episodes 6-7 thru 6-9 I think we can safely assume that Mark Kirkland is not still around.