Reason #1: Lightweight Toilet Flappers
The first problem that you might be experiencing is having a toilet flapper that’s too light.
Your toilet flapper is the door that opens to let the water in your tank move down into your toilet bowl. The flapper has a certain weight, so it closes at the appropriate time after you’ve flushed your toilet. If your flapper is too light, it won’t close fast enough after you flush your toilet.
On the inside, this looks like water continuing to enter the bowl even after your flush is over. On your end, this looks like your toilet flushing two times.
Reason #2: Too Much Water In Your Tank
You could also have the problem of having too much water in your tank. Too much water in the tank means that when you go to flush your toilet, there’s an overflow. The extra water continues to pour into the bowl and refills it quickly. This results in your toilet bowl flushing two times.