In TP.down, we have the following code:
if(loopback && (multicast || (dest.equals(msg.getSrc()) && dest.equals(local_addr)))) {
// loop back
return;
}
send(msg)

If loopback=true and !multicast, we still never loop back a unicast message as local_addr will always be false when we use a shared transport (singleton_name != null).

SOLUTION:
Drop the last part of the if-clause:
if(loopback && (multicast || (dest.equals(msg.getSrc()))))

This is in down(), so msg.getSrc() can only be sent by the local member, so the last part is not needed.

Note that this issue doesn't cause erroneous behavior, but unnecessarily sends a unicast message to self, where it could have been looped back.