Validation Of All Input Fields In Array

I have just started learning JavaScript, so I do not know much about it at this moment. I need suggestion regarding validation of input values which are actually in an array. My HTML looks like this:

Link# 1: <input name="url[]" size="80" type="text">
Title# 1:<input name="title[]" size="80" type="text">
Link# 2: <input name="url[]" size="80" type="text">
Title# 2:<input name="title[]" size="80" type="text">
Link# 3: <input name="url[]" size="80" type="text">
Title# 3:<input name="title[]" size="80" type="text">
.
.
.
.
.
And so on. (up to 20)

User can add more input fields by adding "Add" button; I'm using JavaScript for that purpose. By default, only a couple of fields is shown. I want to validate these all fields using a loop or a number of loops, such that an alert appears if any field is left blank and also if any value in title[] array matches another value in that array and same for the second array url[] Examples of working would be something like this:

If Link# 1 is left blank: alert("Link# 1 is empty");
or
If Link# 2 is left blank: alert("Link# 2 is empty");
or
If Title# 1 is left blank: alert("Title# 1 is empty");
or
If Title# 2 is left blank: alert("Title# 2 is empty");
or
If Link# 1 == Link# 2: alert("Link# 1 is same as Link#2");
or
If Link# 1 == Link# 3: alert("Link# 1 is same as Link#2");
or
If Title# 1 == Title# 3: alert("Title# 1 is same as Title#3"); etc, etc.