If you try to add a patron or recipient twice to a routing list, it creates an error. To test: 1. Create a subscription 2. Create a routing list. 3. Click ‚Add recipients‘ to the routing list. 4. Search for a patron, and add them. Then, add the same person again. Part of the error trace: C4::Serials::addroutingmember(): DBI Exception: DBD::mysql::st execute failed: Duplicate entry '2-19' for key 'subscriptionid' at /kohadevbox/koha/serials/routing.pl line 69 at /usr/share/perl5/DBIx/Class/Exception.pm line 77