bug 1461350: Convert duplicate check to assertion
bug 1461350: Convert duplicate check to assertion

Elchi3/kuma,mozilla/kuma,SphinxKnight/kuma,a2sheppy/kuma,escattone/kuma,Elchi3/kuma,SphinxKnight/kuma,jwhitlock/kuma,Elchi3/kuma,mozilla/kuma,Elchi3/kuma,mozilla/kuma,escattone/kuma,SphinxKnight/kuma,jwhitlock/kuma,a2sheppy/kuma,a2sheppy/kuma,jwhitlock/kuma,mozilla/kuma,a2sheppy/kuma,SphinxKnight/kuma,SphinxKnight/kuma,jwhitlock/kuma,jwhitlock/kuma,SphinxKnight/kuma,mozilla/kuma,escattone/kuma,Elchi3/kuma,a2sheppy/kuma