The double vision just means that you 1) purchased a potent product, 2) drank more than you intended to, 3) may not have had strong kava in the past and had a new experience that was not intended. Any high quality/strong kava will lead to some kind of krunken state when a certain amount is drank, whether you call that too much is very subjective. It's not good if you have things that need to be done but for some people (myself included), it is a very desirable state for end of the day relaxation. We've had a very severe drought of strong kava for a couple of years now but it looks like a couple of vendors, BKH included, are sourcing some higher quality root.
So, the best way to avoid that state is to drink kava with less strength -- most Tongans and weaker Fijians, only drink at the end of the day as a relaxant so you can enjoy the krunk, keep the dose low and wait for the effects of the first dose to clear before re dosing. You'll probably want to avoid the good stuff from Vanuatu since those are the real krunkers.