Hi, I guess you have already adjusted the tension of the chain from the handle to the flapper? Have you checked the tank to be sure that the water in the tank is at the correct level. There should be a mark on the inside of the tank?
If you have checked both of these proceed to the following.
There can be a couple of reasons this is happening to you. You can have an object (pill bottle, childs wooden block, etc.) in the trap below your toilet. When the toilet is flushed the object rises shutting off the water. This causes the flapper to rise and thus giving you a double flush.
If this isn't the case try cutting back on the supply water vavle under the tank to half-way open. This slows the refill process and works as the solution in many cases.
If none of the above solutions work you can add a little weight to the underside of the flapper (inner bell). Putting just a little more weight on the flapper will hold the flapper will shorten the flush time and this solution works in some cases also.
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