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

Question

I have recently published my ASP.NET Core application to my host. I am hitting a HTTP Error 500.19.

IIS 8.5 says the issue is:-

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

It also highlights this key add line in my system.webServer config:-

<handlers>   <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule"       </handlers> 

I'm not really sure what to do on this. It looks as though there is a duplicate instance of this, so I have tried renaming this but it still asks to add this again?

Here is my web.config:-

<?xml version="1.0" encoding="utf-8"?> <configuration>    <!--   Configure your application settings in appsettings.json. Learn more at http://go.microsoft.com/fwlink/?LinkId=786380   -->    <system.webServer>   <handlers>     <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified" />   </handlers>   <aspNetCore processPath="%LAUNCHER_PATH%" arguments="%LAUNCHER_ARGS%" stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout" forwardWindowsAuthToken="false"/>   </system.webServer>  <system.net> <defaultProxy useDefaultCredentials="true" > </defaultProxy> </system.net> </configuration> 

Answer 1

The answer above didn't work for me, however DavidG's comment did solve my problem, so going to post as an answer in case it helps someone else.

For me, I was not running it as a sub-application, and a project that had been working for me no issue for over a year suddenly stopped working with this issue. Still not sure what changed. When I commented out or removed the <add name="aspNetCore".../> the error persisted, and then that line got automatically re-added.

To solve the problem, I added <remove name="aspNetCore" /> to the config file, right above the <add name="aspNetCore"... /> entry, and things started working again.

Answer 2

To continue running on IIS EXPRESS, go on root folder where the .sln file stays.

go to delete file from .vs\config\applicationhost.config or save it in a temporary place if you have something there.

Close/Re Open VS Studio, run again, will work.

If you need to add something back from save applicationhost.config, just compare those two, but I don't see what you could have there.