go/build: handle and warn of duplicate GOPATH entries

R=golang-dev, alex.brainman
CC=golang-dev
https://golang.org/cl/5519050

src/pkg/go/build/path.go

@@ -157,6 +157,7 @@ func init() {
 		Path = []*Tree{t}
 	}
 
+Loop:
 	for _, p := range filepath.SplitList(os.Getenv("GOPATH")) {
 		if p == "" {
 			continue
@@ -166,6 +167,21 @@ func init() {
 			log.Printf("invalid GOPATH %q: %v", p, err)
 			continue
 		}
+
+		// Check for dupes.
+		// TODO(alexbrainman): make this correct under windows (case insensitive).
+		for _, t2 := range Path {
+			if t2.Path != t.Path {
+				continue
+			}
+			if t2.Goroot {
+				log.Printf("GOPATH is the same as GOROOT: %q", t.Path)
+			} else {
+				log.Printf("duplicate GOPATH entry: %q", t.Path)
+			}
+			continue Loop
+		}
+
 		Path = append(Path, t)
 		gcImportArgs = append(gcImportArgs, "-I", t.PkgDir())
 		ldImportArgs = append(ldImportArgs, "-L", t.PkgDir())