net: only return unique hosts during hostname lookup on plan 9

TestLookupHost expects that no duplicate addresses are returned. when cs is consulted for a name, e.g net!localhost!1, it will possibly return multiple available paths, e.g. via il and tcp. this confuses the tests.

LGTM=aram
R=jas, 0intro, aram
CC=golang-codereviews
https://golang.org/cl/58120045

src/pkg/net/lookup_plan9.go

@@ -123,6 +123,7 @@ func lookupHost(host string) (addrs []string, err error) {
 	if err != nil {
 		return
 	}
+loop:
 	for _, line := range lines {
 		f := getFields(line)
 		if len(f) < 2 {
@@ -135,6 +136,12 @@ func lookupHost(host string) (addrs []string, err error) {
 		if ParseIP(addr) == nil {
 			continue
 		}
+		// only return unique addresses
+		for _, a := range addrs {
+			if a == addr {
+				continue loop
+			}
+		}
 		addrs = append(addrs, addr)
 	}
 	return