Personally, I figure that even if every container starts out clean and gets managed properly, there's still a chance of introducing impurities when you reuse one needle to pull liquid from several of them, though that chance might be small. The real issue comes from not knowing for sure if all of them are actually clean to begin with. Should any single one have a problem, that same needle might move the issue over to the rest. For that reason, the recommended way involves grabbing a new needle each time you access a different container before switching to yet another for the actual shot. Sure, pulling the whole amount into a single syringe for one injection works fine and lots of folks go that route. Still, in my view the tiny money saved doesn't justify raising the chance of issues, particularly since these substances often skip individual checks for cleanliness on each container and the needle ends up blunted after going through three of them.