http.ListenAndServe handler function executed twice on port 80 [duplicate] - go

Question

This question already has answers here:
HandleFunc being called twice
(4 answers)
Why this simple web server is called even number times?
(1 answer)
Closed 5 years ago.
If I run the following simple http server code on port 8080 everything works as expected. If I run the same code on port 80, by just changing the port, the handler function is executed twice with each request. Why, and how to fix it?
// httptest project main.go
package main
import (
"net/http"
"log"
"fmt"
"html"
)
var count int
func defaultHandler(w http.ResponseWriter, r *http.Request) {
count++
fmt.Fprintf(w, "Hello, %q count=%d", html.EscapeString(r.URL.Path), count)
fmt.Println(count,r.RemoteAddr)
}
func main() {
http.HandleFunc("/", defaultHandler)
log.Fatal(http.ListenAndServe(":8080", nil))
}
If I enter localhost:8080 in the browser, I get a response with a count starting at 1 and increased by 1 with each new request.
If I change the code to port 80 and enter just localhost or localhost:80 in the browser, I get a first response with a count starting at 1 but increased by two with each following request. At the same time the print statement for the console output is executed twice.
Terminal console when running on port 80 with 3 requests:
>go run main.go
1 [::1]:51335
2 [::1]:51335
3 [::1]:51335
4 [::1]:51335
5 [::1]:51335
6 [::1]:51335
The responses in the browser are Hello, "/" count=1, Hello, "/" count=3 and Hello, "/" count=5.
I've been running this locally on Windows 10 with Go version go1.9.2 windows/amd64 and the latest Google Chrome Browser.
However, I detected the issue in a simple web application on a remote Linux server where the code has been compiled with go version go1.9.1 linux/amd64.

Answer 1

i just tried it on my pc with Fiddler open
I noticed when navigating to the url using Google Chrome, the browser makes 2 request
GET / HTTP/1.1
GET /favicon.ico HTTP/1.1
the request for the favicon also gets handled by the defaultHandler, which causes the count to increment
I also tried with firefox and it doesn't send another request for the favicon

Answer 2

Try to log requests. Possibly browser is calling /favicon.ico