# rpc error: code = AlreadyExists desc = 邮箱为 'a@b.com' 的用户已存在

- **ID:** `go/grpc-already-exists-duplicate-resource`
- **领域:** go
- **类别:** data_error
- **验证级别:** ai_generated
- **修复率:** 80%

## 根因

服务端因唯一约束（邮箱、ID 等）被违反而拒绝创建。通常由不带幂等键的重试引起。

## 版本兼容性

| 版本 | 状态 | 引入 | 弃用 |
|------|------|------|------|
| 1.0+ | active | — | — |

## 解决方案

1. **** (90% 成功率)
   ```
   md := metadata.Pairs("idempotency-key", uuid.NewString())
ctx := metadata.NewOutgoingContext(ctx, md)
_, err := client.CreateUser(ctx, req)
   ```
2. **** (85% 成功率)
   ```
   if status.Code(err) == codes.AlreadyExists {
  existing, _ := client.GetUser(ctx, &pb.GetUserReq{Email: req.Email})
  return existing, nil
}
   ```

## 无效尝试

- **** — The existing resource may differ from the intended one; silently accepting hides data bugs (70% 失败率)
- **** — Creates duplicate logical entities and violates business rules (85% 失败率)