Avoid duplicate featurings when migrating
Avoid duplicate featurings when migrating

Duplicates have occured when publishing a new edition of a document:

https://www.pivotaltracker.com/story/show/46707577

This avoids replicating the duplicates

ggoral/whitehall,YOTOV-LIMITED/whitehall,alphagov/whitehall,YOTOV-LIMITED/whitehall,ggoral/whitehall,alphagov/whitehall,askl56/whitehall,hotvulcan/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall,hotvulcan/whitehall,YOTOV-LIMITED/whitehall,alphagov/whitehall,alphagov/whitehall,robinwhittleton/whitehall,YOTOV-LIMITED/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall,robinwhittleton/whitehall,ggoral/whitehall,askl56/whitehall,askl56/whitehall,askl56/whitehall,ggoral/whitehall