Bug 36459: Do not duplicate duedatespec IDs

Bug 36459: Do not duplicate duedatespec IDs

To test;
1. Go to circ/circulation.pl and check an item out to a patron.
2. Backdate it to sometime in the past using the "Specify due date
   (MM/DD/YYYY) : " input
3. A modal appears to "Please confirm checkout". The date is no longer
   populated in that field.
4. You must add the date again here.
5. APPLY PATCH
6. Try again and this time the confimration date should be set
   correctly.
7. Follow the test plan from Bug 18885 to make sure on site checkouts
   still work correctly.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Emily Lamancusa <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>