daemon: don't listen on the same address multiple times

Before this change:

    dockerd -H unix:///run/docker.sock -H unix:///run/docker.sock -H unix:///run/docker.sock
    ...
    INFO[2019-07-13T00:02:36.195090937Z] Daemon has completed initialization
    INFO[2019-07-13T00:02:36.215940441Z] API listen on /run/docker.sock
    INFO[2019-07-13T00:02:36.215933172Z] API listen on /run/docker.sock
    INFO[2019-07-13T00:02:36.215990566Z] API listen on /run/docker.sock

After this change:

    dockerd -H unix:///run/docker.sock -H unix:///run/docker.sock -H unix:///run/docker.sock
    ...
    INFO[2019-07-13T00:01:37.533579874Z] Daemon has completed initialization
    INFO[2019-07-13T00:01:37.567045771Z] API listen on /run/docker.sock

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit d470252e8742867797a58933f7b4ba34494af2ed)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cmd/dockerd/daemon.go

@@ -608,11 +608,17 @@ func newAPIServerConfig(cli *DaemonCli) (*apiserver.Config, error) {
 
 func loadListeners(cli *DaemonCli, serverConfig *apiserver.Config) ([]string, error) {
 	var hosts []string
+	seen := make(map[string]struct{}, len(cli.Config.Hosts))
+
 	for i := 0; i < len(cli.Config.Hosts); i++ {
 		var err error
 		if cli.Config.Hosts[i], err = dopts.ParseHost(cli.Config.TLS, honorXDG, cli.Config.Hosts[i]); err != nil {
 			return nil, errors.Wrapf(err, "error parsing -H %s", cli.Config.Hosts[i])
 		}
+		if _, ok := seen[cli.Config.Hosts[i]]; ok {
+			continue
+		}
+		seen[cli.Config.Hosts[i]] = struct{}{}
 
 		protoAddr := cli.Config.Hosts[i]
 		protoAddrParts := strings.SplitN(protoAddr, "://", 2)