Help the toilet doesn’t Flush completely the first time or second time

The image attached to this answer, shows what the flush valve is.

It typically includes the overflow tube, but more specifically it's the flapper (or a device that controls the flow of water into the bowl).

Measure the tank discharge (flush) tube (the hole the flush valve (flapper) covers), to determine the size flush valve you need. The hole might be slightly larger or smaller then 2", but close enough is close enough.

Don't worry, dual flush toilets aren't any different. It sounds like your Fill Valve (the tall thing on the left) is on its way out. You can do a new one with just big pliers & you'll need to replace the water hose from the wall to the toilet. $30 & 30-minutes, at the most, should get you back to normal. Or, call Roto Rooter if you don't do that stuff. It's common but not too predictable, every 10-years is a good preventative rule.