Port usage contradiction error about certmagic HOT 6 CLOSED

package main

import (
	"errors"
	"fmt"
	"log"
	"net"
	"strings"
)

func main() {
	if err := run(); err != nil {
		log.Fatal(err)
	}
}
func run() error {
	ip, err := GetOutboundIP()
	if err != nil {
		return err
	}

	port := "1965"

	serverLn, err := net.Listen("tcp", net.JoinHostPort(ip, port))
	if err != nil {
		return err
	}
	defer serverLn.Close()
	fmt.Println("own server listening on", serverLn.Addr().String())

	// certmagic logic
	addr := ":" + port
	ln, listenErr := net.Listen("tcp", addr)
	if listenErr == nil {
		ln.Close()
		return errors.New("certmagic: could listen on " + addr)
	}
	fmt.Println("certmagic: could not listen on", addr, listenErr)

	conn, connectErr := net.Dial("tcp", addr)
	if connectErr == nil {
		conn.Close()
		return errors.New("certmagic: could dial " + addr)
	}
	fmt.Println("certmagic: could not dial", addr, connectErr)

	if strings.Contains(listenErr.Error(), "address already in use") {
		fmt.Printf("[WARNING] OS reports a contradiction: %v - but we cannot connect to it, with this error: %v; continuing anyway 🤞\n", listenErr, connectErr)
	}
	return nil
}

// Get preferred outbound ip of this machine
// adapted from https://stackoverflow.com/a/37382208/3207406
func GetOutboundIP() (string, error) {
	conn, err := net.Dial("udp", "8.8.8.8:80")
	if err != nil {
		return "", err
	}
	defer conn.Close()

	localAddr := conn.LocalAddr().(*net.UDPAddr)

	return localAddr.IP.String(), nil
}