s/(\G                  #  Either were we finished the previous time
                          #     (or at the beginning the first time).
       |                  #  Or
      (.)(?!\2))          #  A character ($2) not followed by itself
     (                    #  Capture ($3)
      (.)\4(?!\4)         #  A character ($4) followed exactly once by
+ itself.
     )                    #  End capture ($3)
    /$2($3)/xg            #  Replace with $2 (what preceeded) followed
+ by
                          #  "($3)" (the duplicate).
[download]

It boils down to that if you have exactly two occurances of a character, something else should preceed those chararcters, or they should be at the beginning of the string (or were we left off the previous time).

Abigail