What consequences can occur with multiple calls to the POST method in an application?

Multiple POST method calls in an application can lead to the following consequences:

• Data duplication: if POST creates resources, repeated calls can create several identical records.
• Idempotency issues: POST by definition is not idempotent, so repeated requests can unpredictably change the server's state.
• Server load: redundant requests can increase load and reduce performance.
• Transaction problems: if the operation is not atomic, repeated calls can lead to inconsistent states.

To prevent these issues, the following are often used:

• Mechanisms of unique request identifiers (idempotency keys).
• Server-side checks to ignore repeated requests.
• Redirect after a successful POST (Post/Redirect/Get) to prevent form resubmission by the user.

Example of using an idempotency key in C# ASP.NET Core:

[HttpPost]
public IActionResult Create([FromBody] Item item, [FromHeader(Name = "Idempotency-Key")] string idempotencyKey)
{
    if (RequestCache.HasProcessed(idempotencyKey))
    {
        return RequestCache.GetResponse(idempotencyKey);
    }
    var result = _service.CreateItem(item);
    RequestCache.StoreResponse(idempotencyKey, result);
    return Ok(result);
}