<table>
    <tr id="duplicate">
        <td style="text-align:center;">1</td>
    </tr>
    <tr id="duplicate">
        <td style="text-align:center;">2</td>
    </tr>
    <tr id="duplicate">
        <td style="text-align:center;">3</td>
    </tr>
    <tr id="duplicate">
        <td style="text-align:center;">4</td>
    </tr>
    <tr id="duplicate">
        <td style="text-align:center;">5</td>
    </tr>
</table>

$(document).ready(function() {
    $('#duplicate'):nth-child(n+3).hide();
    alert('123');
});

<tr class="duplicate">
        <td style="text-align:center;">1</td>
    </tr>
    <tr class="duplicate">
        <td style="text-align:center;">2</td>
    </tr>
    <tr class="duplicate">
        <td style="text-align:center;">3</td>
    </tr>
    <tr class="duplicate">
        <td style="text-align:center;">4</td>
    </tr>
    <tr class="duplicate">
        <td style="text-align:center;">5</td>
    </tr>
...

$(document).ready(function() {
    $('.duplicate:gt(2)').hide();
    alert('123');
});

$(document).ready(function() {
    $('.duplicate:gt(2)').hide();
});

$('.duplicate').gt(2).hide();

<table id="duplicate">
    <tr>
        <td style="text-align:center;">1</td>
    </tr>
    <tr>
        <td style="text-align:center;">2</td>
    </tr>
    <tr>
        <td style="text-align:center;">3</td>
    </tr>
    <tr>
        <td style="text-align:center;">4</td>
    </tr>
    <tr>
        <td style="text-align:center;">5</td>
    </tr>

</table>

$(document).ready(function() {
    $('#duplicate tr:nth-child(n+3)').hide();
    alert('123');
});

<table>
    <tr class="duplicate">
        <td style="text-align:center;">1</td>
    </tr>
    <tr class="duplicate">
        <td style="text-align:center;">2</td>
    </tr>
    <tr class="duplicate">
        <td style="text-align:center;">3</td>
    </tr>
    <tr class="duplicate">
        <td style="text-align:center;">4</td>
    </tr>
    <tr class="duplicate">
        <td style="text-align:center;">5</td>
    </tr>

$(document).ready(function() {
 $("table tr:first").nextAll().hide();
));

$(document).ready(function ()
{

    $("table tr").each(function (indx, ob)
    {

        if (indx > 2)
        {
            $(ob).hide();
        }
    })


});​