[Click] Is it possible for a packet to exist twice inside of a router?

[server at mail.utexas.edu]

Is it possible, for example, for the same exact packet to exist in multiple
queues? I ask, because if you were storing your packets in a central location
and then fetching them back when you got to transmit, you'd need to delete
packets from that central location when they were no longer needed.

If there can only be one copy, then a discard from a queue would always delete
packets from that shared memory. If there can be multiple, you'd have to keep a
counter of how many copies exist.

Which is the case?

Thanks!