Sections must only appear once per config file. See the help topic <location> for exceptions.Why?

Configuration Error Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

<configuration>

  <system.net>
    <mailSettings>
      <smtp from="makovetskiyd@yahoo.co.uk">
        <network host="smtp.gmail.com" enableSsl="true" defaultCredentials="false" port="587" userName="****" password="****"/>
      </smtp>
    </mailSettings>
  </system.net>

  <connectionStrings>
    <clear/>
<add name="SqlState" connectionString="Data Source=localhost;DataBase=AnyNameOfServerState;Integrated Security=True" providerName="System.Data.SqlClient"/>
<add name="YourGuruDB" connectionString="Data Source=DIMA-00AA1DA557;Initial Catalog=model;Integrated Security=True"/>
<add name="modelConnectionString" connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename='D:\Documents and Settings\Dima\My Documents\Visual Stu开发者_运维问答dio 2010\WebSites\WebSite10\App_Data\ASPNETDB.MDF';Integrated Security=True;User Instance=True;Trusted_Connection=Yes;" providerName="System.Data.SqlClient"/>
  </connectionStrings>


  <system.web>

<sessionState mode="SQLServer" sqlConnectionString="SqlState" allowCustomSqlDatabase="true" cookieless="false" timeout="20"/>
<compilation debug="true"/>

<authentication mode="Forms">
  <forms name=".MyCookie" loginUrl="~/Registration.aspx" protection="All" timeout="30" path="/">
    <credentials passwordFormat="MD5">
      <user name="****" password="****"/>
    </credentials>
  </forms>
</authentication>

<authorization>
  <deny users="?"/>
  <allow users="*"/>
</authorization>

<roleManager enabled="true" defaultProvider="MyRoleProvider">
  <providers>
    <clear/>
    <add name="MyRoleProvider" applicationName="/" connectionStringName="modelConnectionString" type="System.Web.Security.SqlRoleProvider"/>
  </providers>
</roleManager>

<membership defaultProvider="MyMembershipProvider">
  <providers>
    <clear/>
    <add name="MyMembershipProvider" type="System.Web.Security.SqlMembershipProvider" connectionStringName="modelConnectionString" minRequiredPasswordLength="1" minRequiredNonalphanumericCharacters="0" enablePasswordReset="true" maxInvalidPasswordAttempts="1000" passwordAttemptWindow="4000" enablePasswordRetrieval="true" requiresUniqueEmail="false" passwordFormat="Encrypted" applicationName="/WebSite10"/>
  </providers>
</membership>

<machineKey validationKey="****" decryptionKey="****" validation="SHA1" decryption="AES"/>
<compilation debug="true" targetFramework="4.0">
  <assemblies>
    <add assembly="System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=****"/>
    <add assembly="System.Web.Extensions.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=****"/>
    <add assembly="System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=****"/>
    <add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=****"/>
  </assemblies>
</compilation>

You have two compilation tags

<compilation debug="true"/> 

and

<compilation debug="true" targetFramework="4.0">   
    <assemblies>     
        <add assembly="System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>     
        <add assembly="System.Web.Extensions.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>     
        <add assembly="System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>     
        <add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>   
    </assemblies> 
</compilation> 

The Error below happen when you have the Compilation element twice in your config file. Just take one or the other one out.

It seem that there is two section.

Remove <compilation>

Leave <compilation debug="true" >

I encountered this problem today after installing Ajax toolkit. If you added custom controls before installing ajax toolkit via in web.config as:

<pages>
    <controls> 
             <!--Your custom control class information-->      
             <add tagPrefix=".." namespace="..."/>      
    </controls>
 </pages>

combine it with ajaxToolkit section after deleting your tags as :

<pages>   
      <controls>
        <add tagPrefix="ajaxToolkit" assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" />    
        <!--Your custom control class information-->
        <add tagPrefix="..." namespace="..." />
      </controls>
</pages>