Error : The entry 'asp:System.Web.UI:System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' has already been added

We are working on a web application and the target deployment includes Windows 2003 64-bit. When we run the application on the 64-bit OS, it gives the fo开发者_运维问答llowing error:

The entry 'asp:System.Web.UI:System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' has already been added

The web.config file includes the following:

<pages validateRequest="false" enableEventValidation="false" viewStateEncryptionMode="Never" theme="esl_theme">
    <controls>
        <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
    </controls>
</pages>

And if we remove the line <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>, it seems to work.

Can somebody help to understand the problem and take corrective action? The application does not compile on 32-bit OS without this.

Is this the root web.config from your application or from a subfolder?

Looks like your application inherits that reference from some parent web.config.

Could be from:

• Root web.config
• Machine's web.config

For your 64-bit problem, check if that assembly is compiled/built for 64-bit or not. Not worked much with 64-bit, but that must be the problem. Try referencing the 64-bit version.