An active connection ID is guaranteed to be unique in the Ably system whilst it is active i.e. no other connection will share that connection ID.
However, Ably reserves the right to generate a new connection ID later that may be the same as a previously discarded connection ID (once the connection is closed).
Therefore we advise customers to not use the connection ID as a perpetual unique identifier as it is possible that a connection ID may be used many times.
See the connection ID documentation for more info.