The issue is that this host is defined in /etc/dnsmasq.d/04-pihole-static-dhcp.conf and pihole.toml. Which leads to the doubling.
Pi-hole should have taken care of this during th transition. Did you manually restore 04-pihole-static-dhcp.conf?
As a workaround, transfer all the settings from 04-pihole-static-dhcp.conf to Pi-hole and remove the file from /etc/dnsmasq.d
I have seen two Tricorder logs by now that showed /etc/dnsmasq.d/*-pihole-.conf files with ownership such (e.g. root:root) that FTL can read (= migrate) the files but then not remove them after migration -> a duplicate is created.
The PR linked by @yubiuser is indeed the best fix.
can also fix the symptoms even if the actual cause is fixed by