doubt in validation in IE

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C#

ASP.NET

in javascript validation my button shows alert msg for error . at the same time it goes into code behind page ,But not at all time ,sometimes it happened.in javascript function i gave return false statement .

how can i solve this ?

Help me?

<asp:imagebutton id="ImageButton1" imageurl="~/Images/images/btn-search.jpg" runat="server" onclick="Submitownerid_Click" onclientclick="return OwnerIDvalidation();" xmlns:asp="#unknown" />

function OwnerIDvalidation() {
       var City = document.getElementById("<%=ddldropcity.ClientID%>").value;
       if (City == "Select City") {
           alert("City has to be Select");
           document.getElementById("<%=ddldropcity.ClientID%>").focus();
           return false;
       }
       return true;
}

protected void Submitownerid_Click(object sender, ImageClickEventArgs e)
{

    Cityname = "Select City";
    Owner_ID = txtownerid.Text;
    USERName = string.Empty;
    CustomerActivatedStartdate = string.Empty;
    CustomerActivatedEnddate = string.Empty;
    GetOwnerDetailsByownerid(Owner_ID);
    //txtName.Text = "";

}