backspace and delete characters not working when a textbox is validated for duplicate source code

I have the following javascript function to validate the usercode value entered in the textbox,i am not allowing them to allow duplicate characters like AABC , BBCF.... etc, but the problem is after entering some characters its not allowing to delete the entered characters.

function ValidateUserCode(txtid,e) 
{

    var validateText = document.getElementById(txtid).value;
    var userText = validateText.toUpperCase();
    if (window.event)
    {
        e = window.event;
    }
    if (e.keyCode) {
        code = e.ke开发者_如何学编程yCode;
    }
    else if (e.which) {
        code = e.which;
        characterCode = String.fromCharCode(e.which);
        characterCode = characterCode.toUpperCase();
    }
    if (userText.indexOf(characterCode) < 0)
     {
        return true;
    }
    else {
        return false;
    }
}

This my code on aspx page

<asp:label Text=" Code" runat="server" ID="Label"></asp:label>
 </div>
     <asp:TextBox ID="CodeTextBox" runat="server" Size="6" MaxLength="4"   Style="text-transform: uppercase;" ></asp:TextBox><br />

This my codebehind where i make javascript call

protected void Page_Load(object sender, EventArgs e)
        {
             IntializeEventHandlers();

}

        private void IntializeEventHandlers()
        {

            this.CodeTextBox.Attributes.Add("onkeypress", string.Format("javascript:return ValidateUserCode('{0}',event);",this.CodeTextBox.ClientID));

        }

Here is a working set of code based on what you want to do that will work in multiple browsers, and properly handles the delete/backspace/arrow keys:

function ValidateUserCode(txtid, e) {
    var validateText = document.getElementById(txtid).value;
    var userText = validateText.toUpperCase();
    var code = e.which;

    if (!code && ((e.charCode || e.charCode === 0) ? e.charCode : e.keyCode)) {
        code = e.charCode || e.keyCode;
    }

    var characterCode = String.fromCharCode(code).toUpperCase();

    return (userText.indexOf(characterCode) < 0);
}

Update

Here's the html I used to test this:

<input type="text" id="mytext" onkeypress="return ValidateUserCode('mytext',event);" />

Make sure that the rendered html from your ASP code looks similar to this, and that the id values match.