Eliminate subsequent chain repetitions

[lidel]

Sometimes the chain loops itself (occurs within corpus edges where there is a limited amount of branches to choose), eg:

<meowkov> lidel: kurtzweil, jo my friend ray kurtzweil, my friend ray kurtzweil, my friend ray kurtzweil, my friend neil tyson new epic cosmos :-[

(It should reject second occurrence of my friend ray kurtzweil,)