WCF - An item with the same key has already been added

Question

I keep getting an error when trying to open WCF WebService via http://www.example.com/WS/myExchange.svc:

System.ArgumentException An item with the same key has already been added.

System.ArgumentException: An item with the same key has already been added. at System.Runtime.AsyncResult.End[TAsyncResult](IAsyncResult result) at System.ServiceModel.Activation.HostedHttpRequestAsyncResult.End(IAsyncResult result) at System.ServiceModel.Activation.ServiceHttpModule.EndProcessRequest(IAsyncResult ar) at System.Web.HttpApplication.AsyncEventExecutionStep.OnAsyncEventCompletion(IAsyncResult ar)

When I open the same WS on the localhost, everything works normally.

I don't even know where to start searching for the root of the problem.

Is the problem in IIS configuration, installed modules, project code?

How to find out which item with the same key has already been added? Even if is it in Web.Config? In code? In database? In included modules?

Problem appears only on "/WS/myExchange.svc", other parts of the website work normally.

Advanced settings:

Site bindings:

Answer 1

A possible cause is

1. Your IIS web application's advanced settings indicates "http,https" in "Enabled Protocols".
2. The "Default Web Site"'s "Site Bindings" does not have an https binding type.

See https://stackoverflow.com/a/19440427/6996876