IDENT field in every fragment matches IDENT field in real datagram. Fragments from different datagrams may arrive out of order and still be saved out.
IP may drop fragment because receiver drops whole original datagram. Destination sets timer with every fragment to check lost fragment. If timer ends before all fragments reach, fragment is assumed lost and datagram is removed. Host is assumed to retransmit.
FRAGMENTING A FRAGMENT:
Fragment may checks subsequent network with even smaller MTU. Router segments the fragment to fit. Resulting sub-fragments look just like real fragments. There is no use to reassemble hierarchically as sub-fragments add position in real datagram.