Ignore duplicate triggers

Somehow I managed to construct a request.cgi url with the same trigger two times e.g.:

https://autopkgtest.ubuntu.com/request.cgi?release=kinetic&arch=ppc64el&package=cmake&trigger=python3-defaults/3.10.6-1&trigger=python3.10/3.10.6-1&trigger=python3-stdlib-extensions/3.10.6-1&trigger=python3.10/3.10.6-1

Notice trigger=python3.10/3.10.6-1 appears twice. This resulted in the following error:

Creating nova instance adt-kinetic-ppc64el-cmake-20220818-184455-juju-4d1272-prod-proposed-migration-4 from image adt/ubuntu-kinetic-ppc64el-server-20220818.img (UUID 84cda4a0-0ac8-45bd-8edc-bad81afd1061)...
autopkgtest [19:03:51]: ERROR: "sh -ec mkdir -p /etc/apt/preferences.d; PKGS=""; PKGS="$PKGS $(apt-cache showsrc python3-defaults python3.10 python3-stdlib-extensions python3.10 | awk '/^Package-List:/ { show=1; next } (/^ / && show==1) { print $1 ":*"; next } { show=0 }' |sort -u | tr '\n' ' ')"; printf "Package: $PKGS\nPin: release a=kinetic-proposed\nPin-Priority: 995\n" > /etc/apt/preferences.d/autopkgtest-kinetic-proposed; printf "
Package: *\nPin: release a=kinetic-updates\nPin-Priority: 990\n" >> /etc/apt/preferences.d/autopkgtest-kinetic-proposed; " failed with stderr "W: Unable to locate package python3.10

The last part of which is especially confusing. It might be better to ignore the duplicate trigger if it is exactly the same as the first trigger.