Very Sporadic Go HTTP Error: multiple response.WriteHeader calls

I wrote Kanali which is an open source Kubernetes Ingress/API management tool and for about 1/200k requests I receive the following fatal error:

2017/08/16 12:40:57 http: multiple response.WriteHeader calls
{"level":"error","method":"GET","msg":"unknown error","time":"2017-08-16T12:40:57Z","uri":"/ommitted/path"}
{"level":"fatal","msg":"write tcp4 192.168.2.160:8443-\u003e192.168.4.0:54554: write: broken pipe","time":"2017-08-16T12:40:57Z"}

I'm having a really hard time reproducing it but here is the relevant code. Kanali is a large project but the td;lr is that after this first code snippet is executed, the second code snipped is executed which handles errors.

func (step WriteResponseStep) Do(ctx context.Context, m *metrics.Metrics, c *controller.Controller, w http.ResponseWriter, r *http.Request, resp *http.Response, trace opentracing.Span) error {
	for k, v := range resp.Header {
		for _, value := range v {
			w.Header().Set(k, value)
		}
	}
    closer, str, err := utils.DupReaderAndString(resp.Body)
    if err != nil {
	    logrus.Errorf("error copying response body, response may not be as expected: %s", err.Error())
    }
    trace.SetTag("http.status_code", resp.StatusCode)
    trace.SetTag("http.response_body", str)
	w.WriteHeader(resp.StatusCode)
	if _, err := io.Copy(w, closer); err != nil {
		return err
	}
	return nil
}

later in the code...

if err != nil {

    w.Header().Set("Content-Type", "application/json")

    switch e := err.(type) {
    case utils.Error:
	    logrus.WithFields(logrus.Fields{
		    "method": r.Method,
		    "uri":    r.RequestURI,
	    }).Error(e.Error())

	    errStatus, err := json.Marshal(utils.JSONErr{Code: e.Status(), Msg: e.Error()})
	    if err != nil {
		    logrus.Warnf("could not marsah request headers into JSON - tracing data maybe not be as expected")
	    }
	    w.WriteHeader(e.Status())
	    if err := json.NewEncoder(w).Encode(utils.JSONErr{Code: e.Status(), Msg: e.Error()}); err != nil {
		    logrus.Fatal(err.Error())
	    }
    default:
	    logrus.WithFields(logrus.Fields{
		    "method": r.Method,
		    "uri":    r.RequestURI,
	    }).Error("unknown error")
	    errStatus, err := json.Marshal(utils.JSONErr{Code: http.StatusInternalServerError, Msg: "unknown error"})
	    if err != nil {
		    logrus.Warnf("could not marsah request headers into JSON - tracing data maybe not be as expected")
	    }
	    w.WriteHeader(http.StatusInternalServerError)
	    if err := json.NewEncoder(w).Encode(utils.JSONErr{Code: http.StatusInternalServerError, Msg: "unknown error"}); err != nil {
		    logrus.Fatal(err.Error())
	    }
    }
}

The error message is telling you that WriteHeader is called more than once (either directly or indirectly by a call to Write). The header can only be written to the network once. Both snippets have a call to WriteHeader.