Configuration Error - Sections must only appear once per config file

Question

I'm getting he following exception

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.

Parser Error Message: Sections must only appear once per config file. See the help topic for exceptions. The error shows in the line

<host factoryType="System.Web.Mvc.MvcWebRazorHostFactory, System.Web.Mvc, Version=5.2.7.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />

My web.config file looks like

    <?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <sectionGroup name="system.web.webPages.razor" type="System.Web.WebPages.Razor.Configuration.RazorWebSectionGroup, System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
      <section name="host" type="System.Web.WebPages.Razor.Configuration.HostSection, System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" />
      <section name="pages" type="System.Web.WebPages.Razor.Configuration.RazorPagesSection, System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" />
    </sectionGroup>
  </configSections>
  <system.web.webPages.razor>
    <host factoryType="System.Web.Mvc.MvcWebRazorHostFactory, System.Web.Mvc, Version=5.2.2.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
    <pages pageBaseType="System.Web.Mvc.WebViewPage">
      <namespaces>
        <add namespace="System.Web.Mvc" />
        <add namespace="System.Web.Mvc.Ajax" />
        <add namespace="System.Web.Mvc.Html" />
        <add namespace="System.Web.Optimization" />
        <add namespace="System.Web.Routing" />
        <add namespace="LS.Web" />
        <add namespace="Stimulsoft.Base" />
        <add namespace="Stimulsoft.Report" />
        <add namespace="Stimulsoft.Report.Mvc" />
      <add namespace="Umbraco.Web" /><add namespace="Umbraco.Core" /><add namespace="Umbraco.Core.Models" /><add namespace="Umbraco.Core.Models.PublishedContent" /><add namespace="Umbraco.Web.Mvc" /><add namespace="Examine" /><add namespace="Umbraco.Web.PublishedModels" /></namespaces>
    </pages>
  <host factoryType="System.Web.Mvc.MvcWebRazorHostFactory, System.Web.Mvc, Version=5.2.7.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
  </system.web.webPages.razor>
  <appSettings>
    <add key="webpages:Enabled" value="false" />
  </appSettings>
  <system.webServer>
    <handlers>
      <remove name="BlockViewHandler" />
      <add name="BlockViewHandler" path="*" verb="*" preCondition="integratedMode" type="System.Web.HttpNotFoundHandler" />
    </handlers>
  </system.webServer>
<system.web>
        <compilation targetFramework="4.7.2">
            <assemblies>
                <add assembly="System.Web.Mvc, Version=5.2.7.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
                <add assembly="System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
                <add assembly="netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" />                
            </assemblies>
        </compilation>
    </system.web></configuration>

Answer 1

Your key is in the web.config twice. I've highlighted the lines in red on the image attached. This can often happen after an Umbraco upgrade in my experience. You'll like want to keep the bottom one, as that is for a higher version (5.2.7.0)