main.go

func main() {
     // ...
        
     e.POST("/users", func(c echo.Context) error {
          var params model.User
          user := model.User{}

          if err := c.Bind(&params); !errors.Is(err, nil) {
           return c.JSON(http.StatusBadRequest, err.Error())
          }

          // Check to see if the user already registered
          err = db.Where(&model.User{UUID: params.UUID}).First(&user).Error
          if !errors.Is(err, nil) && !gorm.IsRecordNotFoundError(err) {
           return c.JSON(http.StatusBadRequest, err.Error())
          }
          if user.UUID != "" {
           return c.JSON(http.StatusBadRequest, "The user already registered")
          }

          // Create a new user when not registered
          user = params
          err = db.Create(&user).Error
          if !errors.Is(err, nil) {
           return err
          }
          return c.JSON(http.StatusCreated, user)
    })
    
  // ... 
}