Hi friend,
1. design the layout in your master page.
2.Delete everything in your content page except the first line.
<![CDATA[<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="AdminMasterPage.aspx.cs" Inherits="Yeshasvini.AdminMasterPage" MasterPageFile= %>]]>
<asp:content id="content1" contentplaceholderid="ContentPlaceHolder1" runat="server" xmlns:asp="#unknown">
</asp:content>
3.In that masterpagefile add your master page.
4.and do the operation inside the contentplaceholder in your content page..
Reason:
The master page already contain body,form everything . So again if its in your page means it will show the error......