I have had what I'd consider to be the most common replication setup:
2 database servers, 1 origin, 1 subscriber, 1 replication set. Slony
replication has been working well for months. The situation is
changing. See Node 3 below.

Node 1 - the origin and main application database
Node 2 - the subscriber and main application backup database
Node 3 - a special database that only needs a subset of the main
application tables

I configured Node 2 to allow it to forward.

My thought was that I'd create a second replication set containing
only the subset of tables needed for Node 3 and use Node 2 as the
origin.

When I try to create the set, I get a PGRES_FATAL_ERROR:  duplicate
key violates unique constraint "sl_table_tab_reloid_key"

So it appears I can't have a single table in more than one replication
set? How do I accomplish what I want?