As the title says, when I import QuestDatabase.json, I get double up on questlines.

Okay thanks, I'll get that fixed up for you though I'm away on holiday so I might not get the update out for a while.

As for your question, once you are happy with your quest setup use:
/bq_admin default save

That makes it the default for new worlds

Do you have a before and after copy of your json file? (not the imported one)

This is the one I import: QuestDatabase (before).json
This is how it looks afterwards, found in the Saves folder: QuestDatabase (after).json
The array in "questLines" have been duplicated.

I think I know what's happening here but just to be sure, you didn't have any existing quest lines before importing right?

None. I'm testing with fresh worlds. So basically, creating a world and then importing.

Related, is there a global way to add quests, so they're visible in all newly created worlds automatically?