you have the common problems with non-uniqueness and with extra elements. Your spec is met by:

    SORT(1,2,3,4,5,5,6) = 1,2,3,4,5,6

    SORT(1,2,3,4,5,5,6) = 1,2,3,4,5,6,7

and a bounds checking bug unique to your formal spec because you only check inputs up to the length of the output

    // first 0 elements of input must occur in output
    SORT(1,2,3,4,5,5,6) = empty list

    // first 6 elements of input must occur in output
    SORT(1,2,3,4,5,5,6) = 1,2,3,4,5,7