var count={};
for (var i = 0; i < array.length; i++) {
    if (count[array[i]]) {
        count[array[i]]++;
    }else {
        count[array[i]]=1;
    }
}
for (var key in count) {
    if (count.hasOwnProperty(key)) {
        console.log(key+':'+count[key]+'times');
    }
}

楼上的稍显不完备

function fengxi(array){
    var count={};
    for (var i = 0; i < array.length; i++) {
        if (count[array[i]]) {
            count[array[i]]++;
        }else {
            count[array[i]]=1;
        }
    }
    var out=[];
    for (var key in count) {
        if (count[key]>1) {
            out.push({"key":key,"count":count[key]})
        }
    }
    var rt="输入数组中存在"+out.length+"组重复数字，它们是:\n"
    for (var i=0;i<out.length;i++){
        rt=rt+out[i][key]+"重复了"+out[i][count]+"次\n";
    }
    return rt;
}
console.log( fengxi([1，1，3，3，3]) );

function getSameNumber(array){
    return array.reduce(function(p,n){
        if(!p[n]){
            p[n] = 1;
        }else{
            p[n] += 1
        }
        return p
    },{});
}