Duplicate Header Expires error

[sheikbe]

We are using paid SaaS standard license. When I try to hit Backend service, I am getting below error

{ "fault": { "faultstring": "Duplicate Header \"Expires\"", "detail": { "errorcode": "protocol.http.DuplicateHeader" } } }

I checked the headers from trace and I don't see Expires header is added. I also tried adding AssignMessage policy to remove Expires header explicitly. That also didn't help. Please suggest

[dchiesa1]

Is it possible the backend system is sending two or more response headers with the name "Expires" ?