c# System.ArgumentException error randomly

We get random error in production stating the following:

System.ArgumentException: Item has already been added. Key in dictionary: 'REMOTE_USER'  Key being added: 'REMOTE_USER'     at System.Collections.Hashtable.Insert(Object key, Object nvalue, Boolean add)     at System.Collections.Specialized.NameObjectCollectionBase.BaseAdd(String name, Object value)     at System.Web.HttpRequest.FillInServerVariablesCollection()     at System.Web.HttpServerVarsCollection.Populate()     at System.Web.HttpServerVarsCollection.Get(String name)     at System.Web.HttpRequest.get_Item(String key) 

SERVER_SOFTWARE LOCAL_ADDR REMOTE_USER HTTP_X_FORWARDED_FOR GATEWAY_INTERFACE APPL_PHYSICAL_PATH HTTPS_SECRETKEYSIZE AUTH_USER APPL_MD_PATH CERT_ISSUER ALL_HTTP LOGON_USER CERT_SECRETKEYSIZE HTTPS_SERVER_ISSUER CERT_COOKIE AUTH_PASSWORD HTTPS SERVER_PORT_SECURE AUTH_TYPE HTTP_HOST CERT_SUBJECT

This occurs in 1 particular method only. We debugged the same but were not able to replicate it. Any pointers would be highly appreciated.

Environment: MVC application calling WCF service

In this method, we receive a request from another controller and pass the data received to the view