If you have multiple v6 addresses on an interface the webui errors, stating there are duplicate IPv6 addresses while there are not. The following network interfaces file yields the screenshotted error.

Code:
# network interface settings; autogenerated
# Please do NOT modify this file directly, unless you know what
# you're doing.
#
# If you want to manage parts of the network configuration manually,
# please utilize the 'source' or 'source-directory' directives to do
# so.
# PVE will preserve these directives, but will NOT read its network
# configuration from sourced files, so do not attempt to move any of
# the PVE managed interfaces into external files!
source /etc/network/interfaces.d/*
auto lo
iface lo inet loopback
auto enp36s0f1
iface enp36s0f1 inet manual
ovs_type OVSPort
ovs_bridge vmbr0
ovs_options tag=1 vlan_mode=native-untagged
auto mgmt
iface mgmt inet static
address 10.20.42.3/24
gateway 10.20.42.1
ovs_type OVSIntPort
ovs_bridge vmbr0
ovs_options tag=42
iface mgmt inet6 static
address 2403:5816:1077:c10c::4/64
address fdac:295e:e5bb::4/64
gateway 2403:5816:1077:c10c::1
auto vmbr0
iface vmbr0 inet manual
ovs_type OVSBridge
ovs_ports enp36s0f1 mgmt
