The API may detect and ignore duplicate events. Each event is uniquely identified as a combination of the following data - the App identifier, the User identifier, the Event name and the Event created time. As a result, it is strongly recommended to send a second granularity Unix timestamp in the created_at field.
Duplicated events are responded to using the normal 202 Accepted code - an error is not thrown, however repeat requests will be counted against any rate limit that is in place.