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

$ go version
go version devel +6965b01ea248 20201124070706 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

https://play.golang.org/p/ZK8yhFoVwW9

	e := xml.NewEncoder(os.Stdout)
	e.EncodeToken(xml.StartElement{
		Name: xml.Name{Space: "space", Local: "local"},
		Attr: []xml.Attr{{Name: xml.Name{Local: "xmlns"}, Value: "spaceAttr"}},
	})
	e.Flush()
	// <local xmlns="space" xmlns="spaceAttr">

What did you expect to see?

One "xmlns" attribute.

What did you see instead?

Two "xmlns" attributes.

While the provided example is obviously very contrived this can happen in the real world (without a user doing something silly like manually adding a second xmlns attribute) in the situation where the user decodes some XML, makes modifications to the tokens, then encodes those tokens somewhere else (probably a fairly common use case). The decoder will add the "xmlns" attribute and fill out Name, then the encoder will duplicate it. The user would have to manually remove xmlns before encoding, or use RawToken (but this is not an option if the decoder is an arbitrary xml.TokenReader so this isn't always possible). Skipping xmlns attributes on marshal if start.Name is set is a simple patch that would fix this issue.