Using the API to create a new ticket sometimes returns an existing ticket

Hi,

I have a weird issue that I can't manually reproduce, but it rarely happens, and I have no idea why.
I have a script that creates a new ticket (POST /api/v2/tickets.json) multiple times a day and in most cases, a new ticket is created every time (code 201). 

However, in some cases, no ticket is created and the response contains a similar ticket that I made earlier (code 200).

The response body object for both POST requests is even different in both cases.

When it creates a ticket, the response is:

{
  "ticket": {
    "subject": "Subject content",
    "updated_at": 1716929554,
    "organization_id": "91********",
    ...
    "id": "12345678",
    "status": "open",
    "requester_id": "51********",
    "assignee_id": "51********"
  },
  "audit": {
  	...
  }
}

and when it doesn't, the response is:

{
  "tickets": [
    {
      "subject": "Subject content",
      "updated_at": 1716929554,
      "organization_id": "91********",
      ...
      "id": "12345678",
      "status": "open",
      "requester_id": "51********",
      "assignee_id": "51********"
    }
  ]
}

As I mentioned, I can't manually reproduce it but I suspect it's related to the “external_id” field which is identical in both cases. Maybe this field or any other similarity between the requests causes Zendesk to think it's a duplicate and not create a new ticket.
But if that was the case, why is it happening very rarely?

Does anyone have an idea what could cause this behavior?

Thanks