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Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to 'ASPxHttpHandlerModule'
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Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to 'ASPxHttpHandlerModule'

Error Description

This error occurs if you create a subfolder in an IIS folder structure and one of the parent folders contains another web.config file with registration of the same handler or module. For instance, you deploy your application to the c:\inetpub\wwwroot\myAppFolder folder, but the c:\inetpub\wwwroot folder contains another web project (or just a web.config file) where the ASPxHttpHandlerModule is registered.

Solution

Research the folder structure to find the cause of the issue. As a workaround, remove ASPxHttpHandlerModule before registering it in web.config:

<system.web> 
    <httpModules>
      <add type="DevExpress.Web.ASPxHttpHandlerModule, DevExpress.Web.v24.1, Version=24.1.1.0, Culture=neutral, PublicKeyToken=b88d1754d700e49a" name="ASPxHttpHandlerModule" />
    </httpModules> 
</system.web>  
<system.webServer>  
    <modules>
      <add type="DevExpress.Web.ASPxHttpHandlerModule, DevExpress.Web.v24.1, Version=24.1.1.0, Culture=neutral, PublicKeyToken=b88d1754d700e49a" name="ASPxHttpHandlerModule" />
    </modules>
</system.webServer>