检查数据库中的现有记录时出现意外行为

我有一个带有 Ember 前端（在 VSCode 中创建）的 ASP.NET CORE 2.1 API 后端。我正在关注 Embercasts 的在线视频教程以进行新用户注册。检查重复用户的逻辑工作不正常：

if (context.Users.Where(u => u.Username.Equals((string) value, StringComparison.OrdinalIgnoreCase)).Count() > 1)
{
     return new ValidationResult("Username is already taken", new [] { "Username" });
}

出于某种原因，上面的代码允许在报告重复用户之前正好有一个重复用户。换句话说，没有达到返回新的 ValidationResult 提醒用户重复记录的代码。我认为这可能与一些奇怪的数组索引问题有关，其中第一条记录位于位置 0。在测试该假设后，我被证明是正确的。以下代码可防止重复，同时仍允许创建一条记录：

if (context.Users.Where(u => u.Username.Equals((string) value, StringComparison.OrdinalIgnoreCase)).Count() > 0)
{
     return new ValidationResult("Username is already taken", new [] { "Username" });
}

有谁知道为什么会这样？任何帮助表示赞赏。

LibraryApi\Model\User.cs

using System;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using JsonApiDotNetCore.Models;

namespace LibraryApiNew.Models
{
    public class User : Identifiable
    {
        [Attr("email"), UniqueEmail, Required(AllowEmptyStrings = false)]public string Email {get; set; }
        [Attr("username"), UniqueUsername, Required(AllowEmptyStrings = false)]public string Username { get; set; }
        [Attr("password"), NotMapped, Required(AllowEmptyStrings = false), Compare("PasswordConfirmation")]public string Password { get; set; }
        [Attr("password-confirmation"), NotMapped, Required(AllowEmptyStrings = false)]public string PasswordConfirmation { get; set; }
        public string PasswordHash { get; set; }
    }

    public class UniqueUsername : ValidationAttribute
    {
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            var context = (AppDbContext) validationContext.GetService(typeof(AppDbContext));

            if (context.Users.Where(u => u.Username.Equals((string) value, StringComparison.OrdinalIgnoreCase)).Count() > 0) // Why?
            {
                return new ValidationResult("Username is already taken", new [] { "Username" });
            }

            return ValidationResult.Success;
        }
    }

    public class UniqueEmail : ValidationAttribute
    {
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            var context = (AppDbContext) validationContext.GetService(typeof(AppDbContext));

            if (context.Users.Where(u => u.Email.Equals((string) value, StringComparison.OrdinalIgnoreCase)).Count() > 0) // Why?
            {
                return new ValidationResult("Email is already taken", new [] { "Email" });
            }

            return ValidationResult.Success;
        }
    }
}