Duplicate Id in mongodb with go

I'm trying to code a simple web app in Go using Mongodb.
I've created a minimalistic simple Model / Controller setup.
I can create new user using POST with url "/user" with data such as '{"pseudo": "bobby1"}'. The user is created. However, when looking inside Mongodb shell, I get:

{ "_id" : ObjectId("5616d1ea56ca4dbc03bb83bc"), "id" : ObjectId("5616d1ea5213c64824000001"), "pseudo" : "bobby2"}

The "id" field is the one coming from my struct and the "_id" field is the one from Mongodb. From looking at different sample code, it looked like I could use myself the one from Mongodb but I can't find how to do it.. ><
Since the "id" is only used by me, I can't find user by their ID since I do not have that one...
More over, when I do a GET /user, it returns the full list of user, and I only get:

{"id":"5616d1ea5213c64824000001","pseudo":"bobby2"

Not the "real" Mongodb Id...

Thanks,

Here's the code:

model/user.go

const userCollection = "user"

// Get our collection
var C *mgo.Collection = database.GetCollection(userCollection)

// User represents the fields of a user in db
type User struct {
	Id     bson.ObjectId `json:"id"i bson:"_id,omitempty"`
	Pseudo string        `json:"pseudo" bson:"pseudo"`
}

// it will return every users in the db
func UserFindAll() []User {
	var users []User
	err := C.Find(bson.M{}).All(&users)
	if err != nil {
		panic(err)
	}
	return users
}

// UserFIndId return the user in the db with
// corresponding ID
func UserFindId(id string) User {
	if !bson.IsObjectIdHex(id) {
		s := fmt.Sprintf("Id given %s is not valid.", id)
		panic(errors.New(s))
	}
	oid := bson.ObjectIdHex(id)
	u := User{}
	if err := C.FindId(oid).One(&u); err != nil {
		panic(err)
	}
	return u
}

// UserCreate create a new user on the db
func UserCreate(u *User) {
	u.Id = bson.NewObjectId()
	err := C.Insert(u)
	if err != nil {
		panic(err)
	}
}

controller/user.go

/ GetAll returns all users
func (us *UserController) GetAll(w http.ResponseWriter, request *http.Request) {
	//	u := model.User{
	//		ID:     "987654321",
	//		Pseudo: "nikko",
	//	}
	users := model.UserFindAll()
	bu, err := json.Marshal(users)
	if err != nil {
		fmt.Printf("[-] Error while marshaling user struct : %v\n", err)
		w.Write([]byte("Error Marshaling"))
		w.WriteHeader(404)
		return
	}

	w.Header().Set("Content-type", "application/json")
	w.WriteHeader(200)
	fmt.Fprintf(w, "%s\n", bu)
}

// Get return a specific user according to Id
func (us *UserController) Get(w http.ResponseWriter, request *http.Request) {
	vars := mux.Vars(request)
	ps := vars["id"]
	u := model.UserFindId(ps)
	bu, err := json.Marshal(u)
	if err != nil {
		fmt.Printf("[-] Error while marshaling user struct : %v\n", err)
		w.Write([]byte("Error Marshaling"))
		w.WriteHeader(404)
		return
	}

	w.Header().Set("Content-type", "application/json")
	w.WriteHeader(200)
	fmt.Fprintf(w, "%s\n", bu)
}

func (us *UserController) Post(w http.ResponseWriter, r *http.Request) {
	u := model.User{}
	json.NewDecoder(r.Body).Decode(&u)
	model.UserCreate(&u)
	bu, err := json.Marshal(u)

	if err != nil {
		fmt.Printf("[-] Error PUT user struct : %v\n", err)
		w.WriteHeader(404)
		return
	}

	w.Header().Set("Content-type", "application/json")
	w.WriteHeader(201)
	fmt.Fprintf(w, "%s\n", bu)
}

Looks like you have a stray character in your struct tags:

type User struct {
    Id     bson.ObjectId `json:"id"i bson:"_id,omitempty"`
    Pseudo string        `json:"pseudo" bson:"pseudo"`
}

That i should not exist after json:"id". It should be:

type User struct {
    Id     bson.ObjectId `json:"id" bson:"_id,omitempty"`
    Pseudo string        `json:"pseudo" bson:"pseudo"`
}