Recalls: guard against issuing a recall multiple times

Recalls: guard against issuing a recall multiple times

We provided no protection against a recall being issued multiple times;
if we have a return date that matches the due date of a potential target
circ, then use that as a flag to not issue the recall.

(Possibly should check for due dates <= instead of just exactly equal)?

Signed-off-by: Dan Scott <dscott@laurentian.ca>