pg_createsubscriber: allow duplicate publication names

Hi,

While testing “Support existing publications in pg_createsubscriber”, I noticed that it doesn’t allow the same publication name even when the publications are from different databases.

A simple repro:
```
% createdb db1
% createdb db2
% psql db1 -c 'CREATE PUBLICATION app_pub FOR ALL TABLES'
% psql db2 -c 'CREATE PUBLICATION app_pub FOR ALL TABLES'
CREATE PUBLICATION
CREATE PUBLICATION
% pg_createsubscriber --dry-run --pgdata <data-dir> --publisher-server postgres --database db1 --database db2 --publication app_pub --publication app_pub
pg_createsubscriber: error: publication "app_pub" specified more than once for --publication
```

The rejection is not introduced by this feature, but previously it was not a real problem because the specified publication names would always be created. After this feature, it may become an issue, since the feature intends to use existing publications, and users may have defined publications with the same name in different databases. So the current restriction becomes a limitation of the new feature. I don’t think we should leave this limitation in PG19.

Since pg_createsubscriber already ensures that duplicate database names cannot be specified, we can just remove the duplicate publication name check.

See the attached patch for details.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/