diff --git a/caddyhttp/proxy/proxy_test.go b/caddyhttp/proxy/proxy_test.go
index c7db7a4b6..e96898bd8 100644
--- a/caddyhttp/proxy/proxy_test.go
+++ b/caddyhttp/proxy/proxy_test.go
@@ -13,6 +13,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"reflect"
 	"runtime"
 	"strings"
 	"sync/atomic"
@@ -407,16 +408,19 @@ func TestUpstreamHeadersUpdate(t *testing.T) {
 	replacer := httpserver.NewReplacer(r, nil, "")
 
 	headerKey := "Merge-Me"
-	values, ok := actualHeaders[headerKey]
-	if !ok {
-		t.Errorf("Request sent to upstream backend does not contain expected %v header. Expected header to be added", headerKey)
-	} else if len(values) < 2 && (values[0] != "Initial" || values[1] != replacer.Replace("{hostname}")) {
-		t.Errorf("Values for proxy header `+Merge-Me` should be merged. Got %v", values)
+	got := actualHeaders[headerKey]
+	expect := []string{"Initial", "Merge-Value"}
+	if !reflect.DeepEqual(got, expect) {
+		t.Errorf("Request sent to upstream backend does not contain expected %v header: expect %v, but got %v",
+			headerKey, expect, got)
 	}
 
 	headerKey = "Add-Me"
-	if _, ok := actualHeaders[headerKey]; !ok {
-		t.Errorf("Request sent to upstream backend does not contain expected %v header", headerKey)
+	got = actualHeaders[headerKey]
+	expect = []string{"Add-Value"}
+	if !reflect.DeepEqual(got, expect) {
+		t.Errorf("Request sent to upstream backend does not contain expected %v header: expect %v, but got %v",
+			headerKey, expect, got)
 	}
 
 	headerKey = "Remove-Me"
@@ -425,12 +429,11 @@ func TestUpstreamHeadersUpdate(t *testing.T) {
 	}
 
 	headerKey = "Replace-Me"
-	headerValue := replacer.Replace("{hostname}")
-	value, ok := actualHeaders[headerKey]
-	if !ok {
-		t.Errorf("Request sent to upstream backend should not remove %v header", headerKey)
-	} else if len(value) > 0 && headerValue != value[0] {
-		t.Errorf("Request sent to upstream backend should replace value of %v header with %v. Instead value was %v", headerKey, headerValue, value)
+	got = actualHeaders[headerKey]
+	expect = []string{replacer.Replace("{hostname}")}
+	if !reflect.DeepEqual(got, expect) {
+		t.Errorf("Request sent to upstream backend does not contain expected %v header: expect %v, but got %v",
+			headerKey, expect, got)
 	}
 
 	if actualHost != expectHost {
@@ -447,6 +450,8 @@ func TestDownstreamHeadersUpdate(t *testing.T) {
 		w.Header().Add("Merge-Me", "Initial")
 		w.Header().Add("Remove-Me", "Remove-Value")
 		w.Header().Add("Replace-Me", "Replace-Value")
+		w.Header().Add("Content-Type", "text/html")
+		w.Header().Add("Overwrite-Me", "Overwrite-Value")
 		w.Write([]byte("Hello, client"))
 	}))
 	defer backend.Close()
@@ -470,6 +475,10 @@ func TestDownstreamHeadersUpdate(t *testing.T) {
 		t.Fatalf("Failed to create request: %v", err)
 	}
 	w := httptest.NewRecorder()
+	// set a predefined skip header
+	w.Header().Set("Content-Type", "text/css")
+	// set a predefined overwritten header
+	w.Header().Set("Overwrite-Me", "Initial")
 
 	p.ServeHTTP(w, r)
 
@@ -477,16 +486,19 @@ func TestDownstreamHeadersUpdate(t *testing.T) {
 	actualHeaders := w.Header()
 
 	headerKey := "Merge-Me"
-	values, ok := actualHeaders[headerKey]
-	if !ok {
-		t.Errorf("Downstream response does not contain expected %v header. Expected header should be added", headerKey)
-	} else if len(values) < 2 && (values[0] != "Initial" || values[1] != replacer.Replace("{hostname}")) {
-		t.Errorf("Values for header `+Merge-Me` should be merged. Got %v", values)
+	got := actualHeaders[headerKey]
+	expect := []string{"Initial", "Merge-Value"}
+	if !reflect.DeepEqual(got, expect) {
+		t.Errorf("Downstream response does not contain expected %s header: expect %v, but got %v",
+			headerKey, expect, got)
 	}
 
 	headerKey = "Add-Me"
-	if _, ok := actualHeaders[headerKey]; !ok {
-		t.Errorf("Downstream response does not contain expected %v header", headerKey)
+	got = actualHeaders[headerKey]
+	expect = []string{"Add-Value"}
+	if !reflect.DeepEqual(got, expect) {
+		t.Errorf("Downstream response does not contain expected %s header: expect %v, but got %v",
+			headerKey, expect, got)
 	}
 
 	headerKey = "Remove-Me"
@@ -495,14 +507,28 @@ func TestDownstreamHeadersUpdate(t *testing.T) {
 	}
 
 	headerKey = "Replace-Me"
-	headerValue := replacer.Replace("{hostname}")
-	value, ok := actualHeaders[headerKey]
-	if !ok {
-		t.Errorf("Downstream response should contain %v header and not remove it", headerKey)
-	} else if len(value) > 0 && headerValue != value[0] {
-		t.Errorf("Downstream response should have header %v with value %v. Instead value was %v", headerKey, headerValue, value)
+	got = actualHeaders[headerKey]
+	expect = []string{replacer.Replace("{hostname}")}
+	if !reflect.DeepEqual(got, expect) {
+		t.Errorf("Downstream response does not contain expected %s header: expect %v, but got %v",
+			headerKey, expect, got)
 	}
 
+	headerKey = "Content-Type"
+	got = actualHeaders[headerKey]
+	expect = []string{"text/css"}
+	if !reflect.DeepEqual(got, expect) {
+		t.Errorf("Downstream response does not contain expected %s header: expect %v, but got %v",
+			headerKey, expect, got)
+	}
+
+	headerKey = "Overwrite-Me"
+	got = actualHeaders[headerKey]
+	expect = []string{"Overwrite-Value"}
+	if !reflect.DeepEqual(got, expect) {
+		t.Errorf("Downstream response does not contain expected %s header: expect %v, but got %v",
+			headerKey, expect, got)
+	}
 }
 
 var (
diff --git a/caddyhttp/proxy/reverseproxy.go b/caddyhttp/proxy/reverseproxy.go
index 5038afcb1..b355f5c69 100644
--- a/caddyhttp/proxy/reverseproxy.go
+++ b/caddyhttp/proxy/reverseproxy.go
@@ -252,8 +252,28 @@ func (rp *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) {
 	io.CopyBuffer(dst, src, buf.([]byte))
 }
 
+// skip these headers if they already exist.
+// see https://github.com/mholt/caddy/pull/1112#discussion_r80092582
+var skipHeaders = map[string]struct{}{
+	"Content-Type":        {},
+	"Content-Disposition": {},
+	"Accept-Ranges":       {},
+	"Set-Cookie":          {},
+	"Cache-Control":       {},
+	"Expires":             {},
+}
+
 func copyHeader(dst, src http.Header) {
 	for k, vv := range src {
+		if _, ok := dst[k]; ok {
+			// skip some predefined headers
+			// see https://github.com/mholt/caddy/issues/1086
+			if _, shouldSkip := skipHeaders[k]; shouldSkip {
+				continue
+			}
+			// otherwise, overwrite
+			dst.Del(k)
+		}
 		for _, v := range vv {
 			dst.Add(k, v)
 		}