#APL(NARS), 20 chars, 40 bytes

    {(,2)≡∪+/¨{⍵=w}¨w←⍵}

Test:

      f←{(,2)≡∪+/¨{⍵=w}¨w←⍵}
      f 7,13,9,2,10,2,4,10,7,13,4,9
    1
      f 77,31,5,31,80,77,5,8,8
    0
      f 2,4,6,4,4,4
    0