It is possible for a client publishing through REST to not receive an acknowledgement of receipt from Ably for numerous reasons, which can lead to the client re-publishing a message. It is possible for Ably to identify incorrectly re-sent messages through idempotent publishing. For connections with idempotent publishing enabled, an identifier is attached to each message sent from the client, enabling Ably to detect duplicates of the same message and ignore them.
At present idempotent publishing is disabled by default, however for releases beyond 1.1 it may be enabled by default.
You can enable idempotent publishing via your client options.