[bug] netifd_add_dynamic(): missing duplicate check causes endless restart loop

This issue is related to openwrt/openwrt#21422
(https://github.com/openwrt/openwrt/issues/21422)
where the same symptom was reported and worked around, but the root cause was not identified.
I can confirm this from personal experience – I am running PPPoE with IPv6 and configured wan6 with option device @wan. The result was an endless restart loop. Renaming wan6 to wan_6 resolved the symptom but not the underlying cause.
Root cause (confirmed – PPPoE + IPv6):
netifd_add_dynamic() in ubus.c does not check whether an interface with the same parent_ifname and proto_handler already exists. When wan6 is configured in UCI, dhcpv6.sh calls add_dynamic("wan_6") – netifd creates this without checking for the conflict → endless loop.
Potentially affected scenarios (code-based, not personally verified):

PPPoE + IPv6 (confirmed): dhcpv6.script creates wan_6, conflicts with UCI wan6
DS-Lite: dhcpv6.script creates wan6_4, may conflict with UCI wan6_4
MAP-E / lw4o6: dhcpv6.script creates wan6_4, may conflict with UCI wan6_4
464XLAT: dhcpv6.script creates wan6_4, may conflict with UCI wan6_4
6rd: dhcp.script creates wan_6, may conflict with UCI wan6

These are personal observations from reading the source code – not a professional assessment. Corrections are welcome.

Read netifd_add_dynamic() in ubus.c first, then compare the dynamic-interface creation paths in dhcpv6.sh and dhcp.script. Reproduce the PPPoE plus IPv6 configuration if possible; done means conflicting dynamic interfaces are handled without duplicate creation and the endless restart loop no longer occurs.