SO_REUSEPORT is introduced in BSD4.4, mainly for multicast. The sematic is to allow completely duplicate bindings, but only if each socket that wants to bind the same IP address and port specify this socket option.
The problem is not all systems support this option. Currently, Solaris and Windows seem not support it yet. But when binding to multicast IP address, SO_REUSEADDR is considered equivalent to SO_REUSEPORT.
May check the situation in future release.