We have a Gerber toilet in our master bathroom and when you flush it, it empties the bowl twice.
Is there a way to adjust the flush valve so that it only flushes once per cycle?
Our other toilets, same brand, work as they should.
You need to look at the toilet bowl and tank and observe what exactly is happening.
The tank doesn't really DO anything to "flush" the toilet. You can flush it by pouring enough water into the bowl.
So the question really is, why is there so much water going into the bowl, to flush it twice?
There are multiple causes to this:
Observing the tank and bowl in a flush will give you more details as to the cause of it.
The toilet bowl flushes by water rushing into the bowl, pushing water down the hole and creating a siphon. The tank simply holds the water, dumps it quickly into the bowl, and refills itself.
To narrow the problem down, quickly pour a bucket of water into the toilet and see what happens. If it still happens, then you have a problem in your drain pipes and don't have to keep messing with the stuff in the tank.
Sounds like to me that the flipper is getting wage to the side or is damaged until enough water gets on top of it to push it down so I would check the flipper just to be on the safe side if that doesn't work double check the amount of water in the tank versus how much water is used to actually flush it should be around about one gallon or 1.5 gallons