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The IP identification being unique is only important if packets are fragmented. Multicast packets are assigned special MAC multicast and routing is layer 3 multicast. IGMP doesn't use the IP ID for membership. Per RFC 5225, it has been observed to being constant. It still works, but not conformant with RFC 791. It still works, but the Firewalls like manipulating the IP ID and other stuff to make it harder to follow the packets.
Per RFC 5225, 6.3.3. IP-ID Behavior
The IP-ID field of the IPv4 header can have different change patterns: sequential in network byte order, sequential byte-swapped, random or constant (a constant value of zero, although not conformant with [RFC0791], has been observed in practice).
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