When customer configured DHCP dual-stack single-session, DHCP subscriber in same vlan may login failure due to "dual stack failure - client already on interface".

About DHCP dual-stack single-session configuration, refer below link：

https://www.juniper.net/documentation/us/en/software/junos/subscriber-mgmt-sessions/topics/topic-map/dual-stack-dhcp-access.html

The classification key defines mechanism to be used to identify a dual stack household. 

In case this knob is not configured by default underlying IFL is used as key.

If one vlan has a dhcp subscriber login, the second dhcp subscriber with different mac try to login, dual-stack checking will consider client already on interface.

From jdhcpd traceoption, we can get below error message:

Mar 12 12:14:18.743554 [MSTR][INFO] [default:default][SVR][INET][xe-0/2/0.3221225502][SID=0] jdhcpd_profile_get: Profile_get using regular profile jw-demux

Mar 12 12:14:18.743610 [MSTR][DEBUG][default:default][SVR][INET][xe-0/2/0.3221225502][SID=0] jdhcpd_dual_stack_get_client_from_key: checking for dual-stack peer flags3(8000), ifl ()

Mar 12 12:14:18.743655 [MSTR][INFO] [default:default][SVR][INET][xe-0/2/0.3221225502][SID=0] jdhcpd_dual_stack_accept_client: dual stack failure - client already on interface.

Mar 12 12:14:18.744388 [MSTR][INFO] [default:default][SVR][INET][xe-0/2/0.3221225502][SID=0] jdhcpd_local_server_state_init: Dropping client due to dual stack check failure.

Set classification key to mac-address can resolve this issue.

For dhcp local server example:

# set system services dhcp-local-server dual-stack-group dhcp-ds classification-key mac-address

For dhcp-relay example:

# set forwarding-options dhcp-relay dual-stack-group dhcp-ds classification-key mac-address