before.go

var sleep time.Duration
known := map[string]struct{}{}
for {
	time.Sleep(sleep)
	ips, _ := readIPs("ips.json")
	for _, ip := range ips {
		if _, ok := known[ip]; !ok {
			known[ip] = struct{}{}
			doSomething(ip)
		}
	}
	sleep = 3 * time.Second
}

func readIPs(file string) (ips []string, err error) {
	if data, err := ioutil.ReadFile(file); err != nil {
		return nil, err
	} else if err = json.Unmarshal(data, &ips); err != nil {
		return nil, err
	}
	return ips, err
}

Can you explain the benefits of this a bit? On first glance this seems to add more code?

More code? It's strictly less lines of code, no? It's also better encapsulated with the utility function to readIPs