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How to enable double tap to switch off screen???
Hi @Shenal and welcome to the community.
I'm afraid there is no such setting in Xperia XA. 
Let me know if there is anything else you are wondering about.
As @Roadrunny76 pointed out you can enable this function if you press and hold on a empty space on your home screen > Tap settings > Double-tap to sleep > On
To enable double tap to switch off screen, keep pressed on a free zone on the home screen until further options appear, then press settings wheel on the bottom right corner and there you can find the option.
Translation via Google Translate:
How do I turn off the screen with two touches
Como le hago para apagar la pantalla con dos toques
The option is available. Keep pressed on an empty area of the home screen until options at the bottom of the screen appear. Select settings (bottom right), scroll down and you will find the option you are looking for "double tap to sleep". Slide the button next to it and that's it. After enabling the option, you have to double tap an empty are of your Home screen to work.
How block the screen touching two times t'he screen?
Thank you. IT works. Very well explained.