Fab guys state that contacts and Via12 are the most prone to get defective. If only 1 of -say- 10000 contacts fails, this (single) circuit will fail if it was a single contact. It probably wouldn't matter for a university project (you'd probably get still enough functional chips), for an industrial project, however, the yield increase of a few % by doubling contacts and vias can be quite important.
The necessity for contact/via doubling depends very much on the margin around the contact/via. If this margin is 100nm or lower, doubling is rather advised. If - in some cases - there's actually no area for a second contact or via (e.g. in standard cells), it would be advisable to extend the margin around it - this is sometimes possible and helps the same.