request, _ := http.NewRequest("GET", "http://localhost:8080/", nil)
request.Header.Add("Content-Type", "A")
request.Header.Add("Content-Type", "B")
request.Header.Add("User-Agent", "C")
request.Header.Add("User-Agent", "D")
_, _ = http.DefaultClient.Do(request)

package main

import (
	"log"
	"net/http"
)

func main() {
	log.SetFlags(0)

	go func() { _ = http.ListenAndServe("localhost:8080", new(echo)) }()

	request, _ := http.NewRequest("GET", "http://localhost:8080/", nil)

	request.Header.Add("Content-Type", "A")
	request.Header.Add("Content-Type", "B")
	request.Header.Add("User-Agent", "C")
	request.Header.Add("User-Agent", "D") // Will not be sent!

	log.Println("Client:", request.Header)

	_, _ = http.DefaultClient.Do(request)
}

type echo struct{}

func (echo) ServeHTTP(_ http.ResponseWriter, request *http.Request) {
	request.Header.Del("Accept-Encoding")
	log.Println("Server:", request.Header)
}

Client: map[Content-Type:[A B] User-Agent:[C D]]
Server: map[Content-Type:[A B] User-Agent:[C]]

$ curl -v "https://www.google.com" -H 'User-Agent: 1' -H 'User-Agent: 2'
...
> GET / HTTP/2
> Host: www.google.com
> Accept: */*
> User-Agent: 1
> User-Agent: 2
...

} else if strings.EqualFold(k, "user-agent") {
	// Match Go's http1 behavior: at most one
	// User-Agent. If set to nil or empty string,
	// then omit it. Otherwise if not mentioned,
	// include the default (below).
	didUA = true
	if len(vv) < 1 {
		continue
	}
	vv = vv[:1]
	if vv[0] == "" {
		continue
	}
}