AVR-Chat

I have an application that needs to receive a variable number of
packets in a block of data.
I store them into a block-sized array with a packet-sized offset
multiplied by the packet number..
I need to know when I've received all the packets, but out-of-order
and duplicate packets are possible.

I was thinking of doing this with a bit-field but that seems pretty messy.
Are there other good ways to know when I've received all the packets,
given that they may arrive out of order?