cmd/go: dedup packages in packagesAndErrors

packagesAndErrors function doesn't dedup packages.
As a result, `go list io ./io` prints io package twice.
Same applies to `go build` and `go test`.

* dedup packages.
* add a test for go list

Change-Id: I54d4063979b1c9359e5416e12327cb85c4823a0f
Reviewed-on: https://go-review.googlesource.com/16136
Run-TryBot: Andrew Gerrand <adg@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -1372,6 +1372,18 @@ func TestGoListCmdOnlyShowsCommands(t *testing.T) {
 	}
 }
 
+func TestGoListDedupsPackages(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	tg.run("list", "xtestonly", "./testdata/src/xtestonly/...")
+	got := strings.TrimSpace(tg.getStdout())
+	const want = "xtestonly"
+	if got != want {
+		t.Errorf("got %q; want %q", got, want)
+	}
+}
+
 // Issue 4096. Validate the output of unsuccessful go install foo/quxx.
 func TestUnsuccessfulGoInstallShouldMentionMissingPackage(t *testing.T) {
 	tg := testgo(t)

src/cmd/go/pkg.go

@@ -1604,15 +1604,24 @@ func packagesAndErrors(args []string) []*Package {
 	}
 
 	args = importPaths(args)
-	var pkgs []*Package
-	var stk importStack
-	var set = make(map[string]bool)
+	var (
+		pkgs    []*Package
+		stk     importStack
+		seenArg = make(map[string]bool)
+		seenPkg = make(map[*Package]bool)
+	)
 
 	for _, arg := range args {
-		if !set[arg] {
-			pkgs = append(pkgs, loadPackage(arg, &stk))
-			set[arg] = true
+		if seenArg[arg] {
+			continue
 		}
+		seenArg[arg] = true
+		pkg := loadPackage(arg, &stk)
+		if seenPkg[pkg] {
+			continue
+		}
+		seenPkg[pkg] = true
+		pkgs = append(pkgs, pkg)
 	}
 	computeStale(pkgs...)