Dual Flush keeps refilling automatically?

Hi,

My dual flush mechanism does not seal the water after it fills up the tank following a toilet flush, so water keeps leaking out into the toilet until the water level in the tank drops to a point where the mechanism opens the refill valve and the water gets topped up and stops at the top. This cycle continues indefinitely because the leak continues all the time. Can somebody tell me how to stop the leak? The mechanism is supposed to stop water escaping after the toilet is flushed. I can’t find the name and model of the mechanism but it has two buttons at the top for the option of small and large water quantity.

Many thanks,
AT