If you gather N characters and plan to make them all take the easy quick quest named 'gathering silk' from Bitral npc and avoid 'Population Control' again and again, then you need this little method (requires N+2 characters, the 2 extra characters are not necessarily from extra accounts, but not from the same account) because a little mechanism exists to avoid (almost systematically) to let two characters take the same quest consecutively (probably to reduce competition in the related zones which are not instances)
For the N-1 first characters just make them hail just once each and take the quest if it appears and pass if not. After several passages you will get the quest for them
For the last Nth character you will probably be stuck with 'Population control' which is the quest you do not want
Have N+1th character hail, until he gets 'Gathering silk' but don't take it
(this one did quest once, but i don t think it matters)
Have N+2nd character hail, until he gets 'Population control' but don't take it
(this one never did quest)
Have Nth character hail, he will surely get 'Gathering silk'
Once again, no need to hail more that once in a row with same character (it doesn't reroll anything, to hail from another character rerolls but not completely randomly, it's more like a toggle)