gridview row command fires twice.

this is my aspx gridview code... 
<asp:GridView ID="gv_video" runat="server" CellPadding="4" AutoGenerateColumns="False" BorderColor="black"
   BorderStyle="Solid" BorderWidth="1px" onrowcommand="gv_video_RowCommand" >
     <Columns>  
     <asp:BoundField DataField="ID" HeaderText="ID"/>
     <asp:BoundField DataField="Title" Headertext="Category Name"/>    
     <asp:BoundField DataField="code" HeaderText="Code"> </asp:BoundField>
     <asp:ButtonField CommandName="editfunction" ButtonType="Image" ImageUrl="~/images/edit.gif" Text="Edit" />
     <asp:ButtonField CommandName="deletefunction" ButtonType="Image" ImageUrl="~/images/delete.gif" Text="Delete" />
    </Columns>
    </ ...