"unknown flag" message prints twice

[wimglenn]

Reproducer:

package main

import "os"
import flag "github.com/spf13/pflag"

func main() {
	fs := flag.NewFlagSet("myprog", flag.ExitOnError)
	fs.Parse(os.Args[1:])
}

Example:

$ go run main.go --bogus
unknown flag: --bogus
Usage of myprog:
unknown flag: --bogus
exit status 2

Workaround: use flag.ContinueOnError instead of flag.ExitOnError, and then exit yourself after parse.