How do I remove duplicate ids?

<td id="table_line_0">
<input id="btnAdd0" type="button" value="+" onClick="newLine()">
</td>
<td id="table_line_0">
<input id="btnAdd1" type="button" value="+" onClick="newLine()">
</td>

I need a javascript function to remove the two id (table_line_0)... tks!!