go/build: handle and warn of duplicate GOPATH entries
go/build: handle and warn of duplicate GOPATH entries

R=golang-dev, alex.brainman
CC=golang-dev
http://codereview.appspot.com/5519050

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang