How do i prevent duplicate insertion from one table to another at client side using javascript

How do i prevent duplicate insertion from one table to another at client
side using javascript

This is my table code.The data gets inserted from another table into this
table but with duplicates.I want to insert rows without duplicates.Do i
need to use array and certain other javascript methods? function
onSelect(code,name,phone)
{
var newRow = document.all("listtable").insertRow(-1);
Cell1 = newRow.insertCell(0); Cell1.innerHTML = code;
Cell2 = newRow.insertCell(1); Cell2.innerHTML = name;
Cell3 = newRow.insertCell(2); Cell3.innerHTML = phone;
Cell4 = newRow.insertCell(3); Cell4.innerHTML = " X ";
}