function find_duplicate_in_array(arra1) {  
  var i,  
  len=arra1.length,  
  result = [],  
  obj = {};   
  for (i=0; i<len; i++)  
  {  
  obj[arra1[i]]=0;  
  }  
  for (i in obj) {  
  result.push(i);  
  }  
  return result;  
  }  
var arr = [1, 2, -2, 4, 5, 4, 7, 8, 7, 7, 71, 3, 6];  
console.log(find_duplicate_in_array(arr));  