diff --git a/hijack.go b/hijack.go
index 477dd992..1a2181df 100644
--- a/hijack.go
+++ b/hijack.go
@@ -356,11 +356,24 @@ func (ctx *HijackResponse) Headers() http.Header {
 
 // SetHeader of the payload via key-value pairs.
 func (ctx *HijackResponse) SetHeader(pairs ...string) *HijackResponse {
+	headerIndex := make(map[string]int, len(ctx.payload.ResponseHeaders))
+	for i, header := range ctx.payload.ResponseHeaders {
+		headerIndex[header.Name] = i
+	}
+
 	for i := 0; i < len(pairs); i += 2 {
-		ctx.payload.ResponseHeaders = append(ctx.payload.ResponseHeaders, &proto.FetchHeaderEntry{
-			Name:  pairs[i],
-			Value: pairs[i+1],
-		})
+		name := pairs[i]
+		value := pairs[i+1]
+
+		if idx, exists := headerIndex[name]; exists {
+			ctx.payload.ResponseHeaders[idx].Value = value
+		} else {
+			ctx.payload.ResponseHeaders = append(ctx.payload.ResponseHeaders, &proto.FetchHeaderEntry{
+				Name:  name,
+				Value: value,
+			})
+			headerIndex[name] = len(ctx.payload.ResponseHeaders) - 1
+		}
 	}
 	return ctx
 }