Gridview Checkbox duplicates

I have gridview with checkbox. When i am trying to get checked checkbox , it gives the duplicate(twice) result.

HTML

 <asp:GridView ID="grdProperty" runat="server" AutoGenerateColumns="false" CssClass="Compgv table-hover "
                Style="width: 100%;" DataKeyNames="SID" OnRowDataBound="grdProperty_RowDataBound"
                AllowSorting="true" OnSorting="grdProperty_Sorting">
                <HeaderStyle ForeColor="White" CssClass="GVFixedHeader" />
 <Columns>
                    <asp:BoundField SortExpression="IsListing,SID" DataField="SID" HeaderStyle-HorizontalAlign="Left"
                        Visible="false" HeaderText="SID" HeaderStyle-Width="0%" ItemStyle-CssClass="table_tr"
                        HeaderStyle-CssClass="table_tr" ItemStyle-HorizontalAlign="Left" />
                    <asp:TemplateField HeaderText="Sl No." HeaderStyle-Width="1%">
                        <HeaderTemplate>
                            <a href="javascript:void(0);" style="color: White">Sl No.</a>
                        </HeaderTemplate>
                        <ItemTemplate>
                            <asp:Label ID="lblSRNO" runat="server" Text='<%#(Container.DataItemIndex+1)%>'></asp:Label>
                            <asp:Label ID="lblSID" runat="server" Text='<%#Eval("SID") %>' Visible="false"></asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField SortExpression="IsListing,ParcelNumber" HeaderText="Parcel No"
                        ItemStyle-HorizontalAlign="Left" HeaderStyle-Width="11%">
                        <ItemTemplate>
                            <asp:CheckBox ID="chk_Compare" runat="server"/>
                            <asp:HiddenField ID="hdnId" runat="server" Value='<%#Eval("SID") %>' />
                            <asp:LinkButton runat="server" ToolTip='<%#Eval("AssessedValue") %>' ID="ParcelNumber" Text='<%#Eval("ParcelNumber") %>'
                                ForeColor="#003366" Font-Underline="True"></asp:LinkButton>
                        </ItemTemplate>
                    </asp:TemplateField>
  </Columns>
</asp:GridView>

Jquery

$("#grdProperty input[id$='chk_Compare']:checked").length;

If i have 4 checked checkbox it result 8, If i have 10 it results 20 and so on. Please help me out