file: /proc/sys/net/ipv6/conf/interface/accept_dad
variable: net.ipv6.conf.interface.accept_dad
Official reference

Whether to accept DAD (Duplicate Address Detection). 0: Disable DAD 1: Enable DAD (default) 2: Enable DAD, and disable IPv6 operation if MAC-based duplicate link-local address has been found.

DAD operation and mode on a given interface will be selected according to the maximum value of conf/{all,interface}/accept_dad.

Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.