Revert Oops, that was not a bug.

arbor/communication/communicator.cpp

@@ -198,7 +198,9 @@ void update_remote_connections(const connectivity& rec,
  for (const auto gid: gids) {
  const auto& ext_conns = rec.external_connections_on(gid);
  for (const auto& conn: ext_conns) {
- if(!is_external(conn.source.rid)) throw arb::source_gid_exceeds_limit(gid, conn.source.rid);
+ // NOTE: This might look like a bug, but the _remote id_ is consider locally
+ // in the remote id space, ie must not be already tagged as remote.
+ if(is_external(conn.source.rid)) throw arb::source_gid_exceeds_limit(gid, conn.source.rid);
  gid_ext_connections.emplace_back(conn);
  }
  part_ext_connections.push_back(gid_ext_connections.size());