cmd/go: Dedup package arguments before building.

Fixes #4104.

R=golang-dev, dave, minux.ma
CC=golang-dev
https://golang.org/cl/6639051

src/cmd/go/pkg.go

@@ -671,7 +671,12 @@ func packagesAndErrors(args []string) []*Package {
 	args = importPaths(args)
 	var pkgs []*Package
 	var stk importStack
+	var set = make(map[string]bool)
+
 	for _, arg := range args {
+		set[arg] = true
+	}
+	for arg := range set {
 		pkgs = append(pkgs, loadPackage(arg, &stk))
 	}
 

src/cmd/go/test.bash

@@ -136,6 +136,12 @@ if GOPATH=:$(pwd)/testdata:. ./testgo build go-cmd-test; then
     ok=false
 fi
 
+# issue 4104
+if [ $(./testgo test fmt fmt fmt fmt fmt | wc -l) -ne 1 ] ; then
+    echo 'go test fmt fmt fmt fmt fmt tested the same package multiple times'
+    ok=false
+fi
+
 if $ok; then
 	echo PASS
 else