What version of Go are you using (go version)?

go1.9.2

What did you do?

https://play.golang.org/p/mxGWipaSUGN

package main

import (
	"encoding/hex"
	"os"
)

func main() {
	d := hex.Dumper(os.Stdout)
	d.Write([]byte(`gopher`))
	d.Close()
	d.Close()
}

What did you expect to see?

00000000  67 6f 70 68 65 72                                 |gopher|

What did you see instead?

00000000  67 6f 70 68 65 72                                 |gopher|
gopher|
|

Calling Close twice is obviously a mistake, but it seems we might as well do something more intelligent than what is currently happening. I doubt anyone is relying the current behaviour.

Mailing CL.