Prevent duplicate entry in first column of a gridview using javascript function

Hello,
I am trying to prevent duplicate values in the first column of a gridview that contains a textbox. I have a link button and users click on it to add rows after entering values, I want to do some javascript validation so any new row's first column cannot contain let's say '01' again, if it exists in any previous rows's first column. I am not sure but for some reason when I enter the very first row in the gridview then it alerts about the duplicate value even though there is no other row available. I think I am getting my counters mixed up but not sure how to correct the issue. Here is the relevant markup section from the griview:
<FooterTemplate> <asp:TextBox ID="txtAddDetail" runat="server" ></asp:TextBox> </FooterTemplate>
<FooterTemplate> <asp:LinkButton ID="lbtnAdd" runat="server" CommandName="ADD" Text="Add Code" Width=" ...