409 Conflict

app.post('/api/users', async (req, res) => {
  const { email } = req.body;
  const existingUser = await db.users.findOne({ email });
  
  if (existingUser) {
    return res.status(409).json({ 
      error: 'Conflict', 
      message: 'A user with this email already exists.' 
    });
  }
  
  await db.users.create(req.body);
  res.status(201).json({ message: 'User created' });
});

from fastapi import FastAPI, HTTPException

app = FastAPI()

@app.put("/documents/{doc_id}")
def update_document(doc_id: int, version: int, content: str):
    current_doc = db.get_document(doc_id)
    # Optimistic concurrency control
    if current_doc.version != version:
        raise HTTPException(
            status_code=409, 
            detail="Document has been modified by another user. Fetch the latest version."
        )
    
    db.update_document(doc_id, content)
    return {"status": "updated"}

func updateHandler(w http.ResponseWriter, r *http.Request) {
    err := updateDatabase(r.Context(), data)
    if err == ErrDuplicateKey {
        http.Error(w, "Record already exists", http.StatusConflict)
        return
    }
    w.WriteHeader(http.StatusOK)
}

[HttpPost("register")]
public async Task<IActionResult> Register(UserDto dto)
{
    var exists = await _userService.EmailExistsAsync(dto.Email);
    if (exists)
    {
        return Conflict(new { Message = "Email address is already taken." });
    }
    // ...
    return Ok();
}

curl -i -X POST https://api.example.com/users \
  -H "Content-Type: application/json" \
  -d '{"email": "existing@example.com"}'