Golang http: multiple response.WriteHeader calls

So I am currently writing a login and respectively a signup features for my Go web App and I am attempting to implement a feature that if you don't fill out both the required form fields "username" "password" it will give you an http.Error and then I am attempting to make it http.Redirect yet i get this error when redirecting occurs. http: multiple response.WriteHeader calls Here is my code..

//Check form submission
var u user
if req.Method == http.MethodPost {
    un := req.FormValue("username")
    p := req.FormValue("password")


    //Checking to see if user filled out required fields.
    if un == ""{
        http.Error(w, "Please fill out required fields, you will be redirected shortly.", http.StatusForbidden)
        time.Sleep(3000 * time.Millisecond)
        //http.Redirect(w, req, "/" http.StatusSeeOther)
        return

    }else if p == "" {
        http.Error(w, "Please fill out required fields, you will be redirected shortly.", http.StatusForbidden)
        time.Sleep(3000 * time.Millisecond)
        //http.Redirect(w, req, "/", http.StatusSeeOther)
        return
    }

    c.Value = un
    u = user{un, p}

    dbUsers[c.Value] = u
    http.Redirect(w, req, "/login", http.StatusSeeOther)

    log.Println(dbUsers)
    return
}

I do know that it is because of the multiple http calls within the if/else statement yet I can't quite come up with an alternative. Any help would be greatly appreciated!