When the remote contact closes the tube with a close message, Gabble closes the tube and sends a close message to the remote contact. The second close message is not needed. It is not in the proto-xep:
http://telepathy.freedesktop.org/xmpp/tubes.html

A tube can be closed because:

1/ a local application calls the D-Bus method call CloseTube
 or
2/ the remote contact sends the close message in a <message> stanza

In the first case, it is appropriate to send the close message to the remote contact. In the second case, we don't need to send the close message because the contact already knows that the tube has to be closed since it requested it. This bug is about the second case.

When the unneeded close message arrives, the tube is already closed, so the tube id is no longer known by Gabble and the message is ignored. So this bug is harmless.