var arr = [1,2,2,3,3,4,4,5,1];

function getRepeat( arr ){
    var len = arr.length,
        map={};

    for( var i = 0;i<len;i++ ){
        if(!map[arr[i]]){
            map[arr[i]] = 1;
        }else{
            map[arr[i]] = map[arr[i]]+1;
        }
    }
    
    for( var n in map ){
        if( map[n] == 1){
            return n;
        }
    }
}
console.log( getRepeat(arr) );