Is it safe to write to a Golang map concurrently if the keys are always unique?

package main

import (
    "sync"
    "fmt"
)

var m = make(map[string]string)

var seen = make(map[string]bool)

func main() {
    wg := new(sync.WaitGroup)
    wg.Add(1)
    ch := make(chan string)
    go deduplicate(ch, wg)

    toAdd := []string{"foo", "bar", "baz", "foo"}
    for _, s := range toAdd {
        ch <- s
    }
    wg.Wait()
    fmt.Println(m)
}

func deduplicate(ch chan string, wg *sync.WaitGroup) {
    for s := range ch {
        if seen[s] {
            wg.Done()
            continue
        }
        seen[s] = true
        go write(s)
    }
}

func write(s string) {
    m[s] = "written"   
}

Is the code above safe to use? Note that multiple strings will be written to the map m concurrently, but they are sure to be unique values, so no string will be written more than once.