how to compare the ids in an array to each other when the user is still entering the values of each ID[i]?

i have this code that creates a table and asks the user to enter the values of IDs and makes sure that the id is a number and breaks if the user inputs it 3 times not a number and prevents the user to input an id number that already exists

i only need some help on how to prevent the user all i got out with is just compering the ID[i] with one previous value and not all the previous values

<script>

var i;

var j;

var w;

var con=0;

var ID=new Array(5);

var grade;

var sum=0;

var names=new Array(5);

names[0]="Duaa";

names[1]="Chaza";

names[2]="Ola";

names[3]="Ghina";

names[4]="Hassan";

document.writeln("<table border='1'>");

for(i=0;i<names.length;i++)

{

con=0;

ID[i]=prompt("Enter The Id number of "+names[i],"ID number");

while(isNaN(ID[i])&&con<2)

{

con++;

ID[i]=prompt("Enter The Id number of "+names[i],"ID number");

}

/* This loop */

for(w=0;w<ID.length;w++)

{

if(ID[i]==ID[i-1])

{

window.alert("You Have Already Entered That Id Number!");

break;

}

}

if(con==2)

{

window.alert("You are a loser!");

break;

}

else

{

/*other details and stuff i'm already done with */

}

}