Bug 14973: Fix edit a suggestion and dup found

Bug 14973: Fix edit a suggestion and dup found

The find duplicate call must only be done when the suggestion is new. It
does not make sense to search for a duplicate when the suggestion
already exists.

This patch also fixes a side-effect:
- Create a suggestion using an existing biblio title
- Ignore the warning and save
- Edit again and save
=> BOOM on date

Template process failed: undef error - The given date (18/11/2019) does
not match the date format (iso) at /kohadevbox/koha/Koha/DateUtils.pm
line 168

The dates are not processed and so badly formatted when sent to the
template.

Sponsored-by: BULAC - http://www.bulac.fr/
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>