LP#1624443 - TPAC 500 error after adding duplicate title to Temp List

LP#1624443 - TPAC 500 error after adding duplicate title to Temp List

Credit to gmcharlt, prevents the my list (temporary list) page from
crashing when a user has added a title to the list more than once.
This can happen if they add a title then login, then redo search and
add same title. Per Galen, it looks like an issue with
OpenSRF::MultiSession parallel record retrieval causing some kind of
race condition when there are duplicate entries in the list.

Signed-off by: Cesar Velez <cesar.velez@equinoxinitiative.org>