tpl, err := template.New("main").Parse(`
    {{template "test" .}}
    {{define "test"}}{{end}}
`)

tpl, err = tpl.New("test").Parse(`This is a test`)
// no error

test, err := template.New("test").Parse(`This is a test`)
tpl, err = tpl.AddParseTree("test", test.Tree)
// error

    if t.common != nil
        if old := t.tmpl[name]; old != nil && !parse.IsEmptyTree(old.Root) {
            return nil, fmt.Errorf("template: redefinition of template %q", name)
        }
    }