What does "Replication has detected an existing connection" mean? Why does AMPS close the connection?

When two AMPS instances replicate directly to each other, AMPS has the ability to use a single connection for replication. The 1E-0063 message indicates that AMPS has detected that it can use a single connection, and is closing the redundant connection.

This message is not an error. No corrective action is required. The message is provided at info level to provide an explicit record of the fact that AMPS closed the connection.

Notice that even though a single network connection is used, each instance of AMPS still treats this as an outgoing message stream that pushes messages to the remote instance and an incoming message stream that receives messages from the remote instance. Messages still flow both ways.

If you don't want to see this message in the event logs, you can add event 1E-0063 to the ExcludeErrors block in your logging configuration.