crea lista di elem presenti + di 2 volte

function fantasy(l)
{
    var prev,m={},l1={};
    if(l==null){return;}
    while(prev.prec!=null){ prev=prev.prec;}
    while(prev!=null)
    {   if(m[prev.x]===undefined){m[prev.x]=1; prev=prev.succ;}
        if(m[prev.x]==1){m[prev.x]=2;prev=prev.succ;}
        else{l1={x:prev.x,succ:l1};prev=prev.succ;}
    } return l1;
}