31.3.1 Duplicate Submission

As described here above, the MessageId can be used to prevent duplicate jobs being created. Since MessageId is used as a localid, it should not be longer than 80 characters, and values should be chosen that are unlikely to collide with those of other users.
The MessageId, if used, is the only request characteristic that is used to detect a duplicate. Other request characteristics, such as the destinations or filenames, are not considered. So care must be taken to ensure that the MessageId for each distinct request has a distinct value. A common mistake is to use, as part of the MessageId, a timestamp with a 1-second resolution, which will not be adequate if more than one request is ever submitted within the same second.
To ensure a unique MessageId value unlikely to collide with those of another user, it is recommended that applications include some user-related string (keeping in mind the 80-character limit) in the value, such as a user id, or company URL or name, or a base64-encoded SHA based on some similar user-specific value.
When a collision occurs, it may be helpful to know from the result whether the job submit request was a duplicate or not, and whether it was a collision with a job from the same user or not.

If the collision was with a job from a different user (or if the user associated with the previous submission could not be determined), then the StatusCode will be "1" (failure), and an ErrorCode of 2051 will be included.

If the collision was with a job from the same user, then the result depends on whether Preview was requested.

If Preview was not requested, or if Preview was requested and the preview result is still available, then the StatusCode will be "0" (success) and the pre-existing job number will be returned, since it is presumed that this is a resend of an earlier request for which the result was for some reason not received. But an ErrorCode of 2050 will also be included as an indication that this was not the first time that the MessageId was received.

If Preview was requested but the preview result is no longer available, then the StatusCode will indicate failure, and an ErrorCode of 2052 will be included.

The use of MessageId to prevent repeated Cloud Fax and Notifications API requests from generating duplicate jobs is primarily intended as a safeguard against generating duplicate jobs if a result is lost due to network connection problems.