register notification user do nothing if exists

service/notification_user_service.go

@@ -62,6 +62,10 @@ func GetNotificationUserById(id primitive.ObjectID) (model.NotificationUser, err
 	return getNotificationUserByBsonDocument(bson.D{{"_id", id}})
 }
 
+func GetNotificationUserByToken(token string) (model.NotificationUser, error) {
+	return getNotificationUserByBsonDocument(bson.D{{"token", token}})
+}
+
 func RemoveNotificationUserById(id primitive.ObjectID) error {
 
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
@@ -98,18 +102,29 @@ func UpdateNotificationUser(user model.NotificationUser) (model.NotificationUser
 	return GetNotificationUserById(user.Identifier)
 }
 
+// RegisterNotificationUser adds the given token to the database,
+// if the token already exists, it just returns the user
 func RegisterNotificationUser(token string) (model.NotificationUser, error) {
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
 
-	user := model.NotificationUser{
-		Token: token,
-	}
-
-	r, err := notificationUserCollection.InsertOne(ctx, user)
+	existing, err := GetNotificationUserByToken(token)
 	if err != nil {
-		return model.NotificationUser{}, err
+		if err.Error() == entryNotFoundMessage {
+			user := model.NotificationUser{
+				Token: token,
+			}
+
+			r, err2 := notificationUserCollection.InsertOne(ctx, user)
+			if err2 != nil {
+				return model.NotificationUser{}, err2
+			}
+
+			existing, _ = GetNotificationUserById(r.InsertedID.(primitive.ObjectID))
+		} else {
+			return model.NotificationUser{}, err
+		}
 	}
 
-	return GetNotificationUserById(r.InsertedID.(primitive.ObjectID))
+	return existing, nil
 }