According to an ipv6 dhcp log (Wireshark) example:
https://packetpushers.net/wp-content/uploads/2011/10/dhcpv6_2.jpg
We can see the client's ipv6 address already there before doing DHCP (S.A.R.R.)
because protocol allowed it ? just do a confirmation with DHCP server to keep the existed ipv6 address?