A client identifier must be unique and less than 23 chars.

[mcollina]

As reported by @davedoesdev in #31, the spec for the client identifiers states:

The first UTF-encoded string. The Client Identifier (Client ID) is between 1 and 23 characters long, and uniquely identifies the client to the server. It must be unique across all clients connecting to a single server, and is the key in handling Message IDs messages with QoS levels 1 and 2. If the Client ID contains more than 23 characters, the server responds to the CONNECT message with a CONNACK return code 2: Identifier Rejected.

Moreover, it's common implementation that if the same clientId connects twice to the same server, the oldest one is disconnected.

[mcollina]

This has been reverted in 0.15.